Osteoporosis is a major public health problem in industrialized countries because it is an underlying cause of fractures in postmenopausal women and elderly persons. In this study, we describe the epidemiology of osteoporotic fractures of the hip and vertebrae in Japan. The annual incidence of hip fracture was estimated by a nationwide survey as ranging between 41, 000 and 48, 000 in 1987. The incidence increased with increasing age in both sexes, and the incidence in females was about twice that in males. Regional differences were also found, in that the incidence of hip fracture in females was low in the eastern part of Japan and high in the west. A study of thoracic vertebral fracture incidence by the Radiation Effects Research Foundation (RERF) showed that it increased in all birth cohorts with increasing age in females but did not increase with age in males. The incidence was significantly lower in the younger birth cohorts in both sexes. Based on these descriptive epidemiological data, analytic and interventional studies of osteoporosis are ongoing at RERF to elucidate risk factors and to prevent bone loss and osteoporotic fractures.