result from the retrospective design, which makes it difficult to reliably establish the participants’ past history of risk-factor exposure. To address this issue, large prospective cohort studies of adults, involving up to 500,000 participants, are being planned (Meade et al. 2002). These studies will document exposure to environmental risk factors and determine known and still-to-be-elucidated genetic risk factors for each participant. Thus, such studies have the potential to establish prospectively the effects of risk factors, genotype, and risk factor × genotype interactions on later disease outcomes.
