The Prevention and Reduction of Obesity through Active Living (PROACTIVE) is a randomized controlled trial to evaluate the effectiveness of a behaviorally based physical activity and diet composition program to prevent and reduce obesity and related co-morbidities in a primary health care setting. We randomized 491 abdominally obese men and women 25-75 years of age who were patients of primary care physicians to either a usual care group (N=242), or a behavioral intervention group (N=249). Those in usual care received general advice from the physician regarding the merits of physical activity and a healthy diet as a strategy for obesity reduction. Those in the behavioral intervention group received an individually designed counseling program from a specially trained Health Educator, with respect to physical activity, diet and obesity reduction. The study was designed to provide 95% power in both men and women to detect a 2% (2 cm) difference in waist circumference and 80% power to identify a 15% reduction in the prevalence of the metabolic syndrome, the two primary outcomes. Primary analyses will compare study groups on mean outcome measures at 24 months post-randomization, be adjusted for baseline value of the outcome measure, and be conducted separately for men and women. PROACTIVE is the first behavioral intervention study to assess the effects of physical activity and diet on abdominal obesity and associated metabolic risk factors in a primary health care setting, includes a generalized sample of men and women, and examines long-term (24 months) effects.