Very little information is available to direct the prevention or management of osteoporosis in men. The Osteoporotic Fractures in Men (MrOS) Study is a prospective cohort study designed to examine the extent to which fracture risk is related to bone mass, bone geometry, lifestyle, anthropometric and neuromuscular measures, and fall propensity, as well as to determine how fractures affect quality of life in men. The study is also designed to understand how osteoporosis is related to prostate disease. At baseline, participants completed questionnaires regarding medical history, medications, physical activity, diet, alcohol intake, and cigarette smoking. Objective measures of anthropometric, neuromuscular, vision, strength, and cognitive variables were obtained. Skeletal assessments included DEXA, calcaneal ultrasound, and vertebral radiographs. Vertebral and proximal femoral QCT was performed on a subset (65%). Serum, urine, and DNA specimens were collected. After the baseline assessments, a questionnaire is mailed to participants every 4 months to ascertain incident falls, fractures, prostate cancer, and deaths. After an average of 4.5 years, participants are scheduled to return for a second comprehensive visit. Men were eligible if > or =65 years. 5995 men enrolled with a mean (+/-SD) age of 73.7 (+/-5.9) years, 11% of which were minorities. Most rated their health as good/excellent. Few were current smokers, although 59% had smoked previously, and 35% reported no alcohol intake, while 47% consumed at least 2 drinks per week. The mean (range) body mass index was 26.9 kg/m2 (17-56). A non-traumatic fracture after age 50 was reported by 17% of the cohort. The MrOS cohort should provide valuable information concerning the determinants of fracture in men and should help set the stage for the development of effective methods to identify those at risk.