The Mexico City Prospective Study (Estudio Mexicano deCohorte Para Enfermedades Cro´nicas en una Poblacio´nMetropolitana) is a blood-based cohort study involvingfollow-up of 150000 adults (50000 men and 100000women) who were aged at least 35 years when recruited in1998–2004. The study resulted from discussions in the early1990s about how best to measure the changing health effects oftobacco in Mexico. These discussions evolved into a plan toestablish a prospective cohort study that could investigate notonly the health effects of tobacco but also those of blood lipidsand various other risk factors. Ideally, such a study should bebig enough to assess the effects of risk factors not only overallbut also in specific circumstances (e.g. at particular ages andseparately in each sex) and at different levels of other riskfactors (to examine possible causal interactions). In Mexico,there have been substantial decreases in overall mortality frominfectious diseases over the past few decades, leaving thechronic diseases of middle age as the main causes of prematuredeath.