ond through 16th floors. The lobby floor, which is slightly above street level, included a registration desk, a counter for the sale of newspapers and sundries, several shops and airline offices, ladies’ and men’s rooms and two restaurants and lounges. Meeting rooms were on the first and 18th floors. The air-conditioning system consisted of two water chillers in the subbasement from which chilled water was circulated to approximately 60 air-handling units in the building.

A case was considered Legionnaires’ disease if it met clinical and epidemiologic criteria. The clinical criteria required that a person have onset between July 1 and August 18, 1976, of an illness characterized by cough and fever (temperature of 38.9°C or higher) or any fever and chest x-ray evidence of pneumonia. To meet the epidemiologic criteria, a patient either had to have attended the American Legion Convention held July 21–24, 1976, in Philadelphia or had to have entered Hotel A between July 1 and the onset of the illness. A person was considered to have had Broad Street pneumonia if he met the clinical but not the epidemiologic criteria for Legionnaires’ disease but had been within one block of Hotel A between July 1 and the onset of illness. A person was considered to have seroconversion if there was a fourfold or greater rise in titer to 1:64 or higher by indirect fluorescent-antibody measurement using as antigen the unnamed gram-negative bacterium implicated as the cause of Legionnaires’ disease.³ A person was considered seropositive if he had a titer of 1:128 or greater by indirect fluorescent-antibody assay.

Information about cases was obtained through active and passive surveillance. After recognition on August 2 that an outbreak had occurred among those attending the American Legion Convention, the Pennsylvania Department of Health alerted local health officials to help in the investigation. State health officials also notified the Pennsylvania Medical Society, the Pennsylvania Osteopathic Association and the Hospital Association of Pennsylvania of a potential statewide epidemic. They requested co-operation in reporting potential cases, and, because of the statewide nature of the problem, the Pennsylvania Department of Health was made the center for planning and data collection.

Public-health nurses were instructed to search hospitals in their districts for hospitalized Legionnaires. They made daily rounds at local hospitals and reported date of onset of illness, clinical description, association with the convention and existence of secondary spread of illness. A telephone hot line was established in Philadelphia, and the public invited to report possible epidemic cases. News reports were scanned to identify additional cases.

Eight epidemiologic surveys are discussed in this paper.

Hotel-guest survey. To assess the rate at which illness meeting the clinical criteria was occurring in hotel guests other than Legionnaires in Philadelphia, a telephone randomized survey was made of guests registered at four hotels between July 6 and August 7. The hotels surveyed included Hotels A and B as well as a center city hotel where a few Legionnaires stayed (Hotel C), and a hotel on the periphery of the city where no Legionnaires stayed (Hotel D). The overall completion rate was 67 per cent; however, there were considerable differences in completion rates within time periods for each hotel and between hotels. An additional group of 80 persons not attending the convention who were registered at Hotel A on July 23 were also selected for interview.

Hotel-employee survey. To determine if illness was affecting employees of the headquarters hotel, a review was made of unscheduled employees’ absences since July 1, and each employee who missed two or more days was interviewed concerning illness that might meet the case criteria. In addition, an approximate 25 per cent randomized sample of the estimated 400 employees of Hotel A was selected for telephone questioning. Also, an attempt was made to interview all persons who worked in the lobby.

Roommate survey. Fifty-nine roommates of 52 patients and 69 roommates of 68 control Legionnaires at Hotel A were queried about illness in the interval from July 21 to August 9, 1976.

Hospital survey. To determine if illness meeting the clinical criteria of disease was occurring apart from the American Legion Convention and Hotel A, emergency-room and admission records of patients seen from July 1 to August 9, 1976, in three hospitals serving center-city Philadelphia (Graduate Hospital, Pennsylvania Hospital and Thomas Jefferson University Hospital) were reviewed.

Pneumonia and influenza deaths. Deaths caused by pneumonia and influenza are routinely reported to the Philadelphia City Health Department as part of the national influenza surveillance system. Review was made of reported deaths in the interval from June 4 through September 24 for the years 1974, 1975 and 1976 among Philadelphia residents.

Legionnaire census. On August 9 a packet of two-page questionnaires regarding activities at the convention and subsequent illness was delivered to the commanders of each of the 1002 local American Legion posts in Pennsylvania. Each commander was asked to identify persons in or associated with his post who had attended the convention, to deliver a form to each person and to retrieve completed questionnaires; 3683 forms were returned by Legionnaires who had attended the convention. Of the 3580 who listed their convention status, 1849 (51.6 per cent) were delegates. Because 2274 delegates voted at the convention, it may be estimated that approximately 4400 persons attended the convention The responses of persons who reported having been well since the convention were compared with those of patients.

Case-control survey No. 1. A case-control survey, designed after completion and analysis of the Legionnaire census, was intended for all surviving male Legionnaires on the case list and a randomized sample of 202 men chosen from the Legionnaire census who indicated that they had been well since the convention (controls). Interviews were completed by telephone on August 17 with 147 control Legionnaires (73 per cent) and 113 case Legionnaires (91 per cent).

Case-control survey No. 2. An attempt was made to survey 56 case and 56 control Legionnaires in December, five months after the epidemic. The patients chosen were the surviving male delegates who had been hospitalized and were known to have had a temperature of 38.9°C or higher and radiographic evidence of pneumonia. Controls were matched for age and sex. Fifty-two case and age-matched control pairs were interviewed in person.

Various parts of the inanimate environment of Hotel A were sampled in August to determine if changes in the physical environment of the hotel might have taken place in temporal association with the Legionnaires’ convention. Weather records for Philadelphia were reviewed. Selected autopsies were attended by medical epidemiologists to supervise collection of appropriate specimens. Interviews were conducted with residents and workers in the area of Hotel A, with persons attending several other conventions and with officials of the American Legion. Two patients with Legionnaires’ disease who had had brief exposure were brought back to the scene of the convention and “walked through” their activities at the time. Medical epidemiologists attempted to interview in person and examine all suspect cases identified as of August 3. Hospital medical records of 94 and 147 hospitalized patients were obtained and reviewed.

The Bureau of Laboratories of the Pennsylvania Department of Health co-ordinated local collection of specimens from all persons. Details of the laboratory aspects of the investigation are presented in a companion paper.³