OBJECTIVE - The Diabetes Prevention Program (DPP) is a 27-center randomized clinical trial designed to evaluate the safety and efficacy of interventions that may delay or prevent development of diabetes in people at increased risk for type 2 diabetes. RESEARCH DESIGN AND METHODS - Eligibility requirements were age ≥25 years, BMI ≥24 kg/m2 (≥22 kg/m2 for Asian-Americans), and impaired glucose tolerance plus a fasting plasma glucose of 5.3-6.9 mmol/l (or ≤6.9 mmol for American Indians). Randomization of participants into the DPP over 2.7 years ended in June 1999. Baseline data for the three treatment groups - intensive lifestyle modification, standard care plus metformin, and standard care plus placebo - are presented for the 3,234 participants who have been randomized. RESULTS - Of all participants, 55% were Caucasian, 20% were African-American, 16% were Hispanic, 5% were American Indian, and 4% were Asian-American. Their average age at entry was 51 ± 10.7 years (mean ± SD), and 67.7% were women. Moreover, 16% were <40 years of age, and 20% were ≥60 years of age. Of the women, 48% were postmenopausal. Men and women had similar frequencies of history of hypercholesterolemia (37 and 33%, respectively) or hypertension (29 and 26%, respectively). On the basis of fasting lipid determinations, 54% of men and 40% of women fit National Cholesterol Education Program criteria for abnormal lipid profiles. More men than women were current or former cigarette smokers or had a history of coronary heart disease. Furthermore, 66% of men and 71% of women had a first-degree relative with diabetes. Overall, BMI averaged 34.0 ± 6.7 kg/m2 at baseline with 57% of the men and 73% of women having a BMI ≥30 kg/m2. Average fasting plasma glucose (6.0 ± 0.5 mmol/l) and HbA(1c) (5.9 ± 0.5%) in men were comparable with values in women (5.9 ± 0.4 mmol/l and 5.9 ± 0.5%, respectively). CONCLUSIONS - The DPP has successfully randomized a large cohort of participants with a wide distribution of age, obesity, and ethnic and racial backgrounds who are at high risk for developing type 2 diabetes. The study will examine the effects of interventions on the development of diabetes.