The SUpplementation en VItamines et Minéraux AntioXydants (SU.VI.MAX) Study is a randomized, double-blind, placebo-controlled, primary-prevention trial designed to test the efficacy of daily supplementation with antioxidant vitamins (vitamin C, 120 mg; vitamin E, 30 mg; and beta-carotene, 6 mg) and minerals (selenium, 100 microg; and zinc, 20 mg) at nutrition-level doses (one to three times the daily recommended dietary allowances) in reducing several major health problems in industrialized countries, especially the main causes of premature death, cancers and cardiovascular diseases. The present report describes the design, implementation, and baseline characteristics of participants in this 8-year cohort study, which started in 1994 in France; 12,735 eligible subjects (women aged 35-60, and men aged 45-60) were included in 1994 and will be followed for 8 years. Participants undergo a yearly visit consisting, every other year, of either biological sampling or clinical examination. They also regularly provide information on health events and dietary intake by filling out computerized questionnaires using the Minitel Telematic Network. Data on baseline characteristics of the participants suggest that the present sample is close to the national population in terms of geographic density, socioeconomic status, and the distribution of various major risk factors for the diseases under study. The choice of the study population should allow the results of this trial to apply to adult populations of both sexes in France and other industrialized countries.