ize on these complementary opportunities (e.g., the advanced use of new formulations in Europe and variations in background breast cancer incidence rates). The potential to maximize future knowledge by planned coordination of international research should be explored most immediately in the United Kingdom and the United States.

The cost of long-term prospective studies is driven by the number of people who must be tracked and the number of years tracking must be maintained. For the purposes of these studies, breast cancer is relatively uncommon; thus, large numbers of women must be followed —making studies of breast-cancer incidence unavoidably expensive. Research costs can be contained by careful selection of study populations and sources of information about outcome measures. National, regional, or institutionally defined populations with linked record systems offer the best opportunity for relatively low-cost collection of information about exposure and, in some cases, outcomes. Population-based cancer registries such as the Surveillance, Epidemiology, and End Results (SEER) program provide an important resource for identification of cancer cases. It may be more cost-efficient to provide support to maintain the integrity of these sources of subjects, information, and events than to establish entirely new, dedicated recruitment, collection, and follow-up systems. In the United States, particular consideration should be given to the populations of large, comprehensive, closed health maintenance organizations within SEER collection areas.