This study was designed to compare the bone mineral density (BMD) and the prevalence of osteoporosis in community-dwelling women (aged 50-89) living in Northern (Beijing) and Southern (Hong Kong) China. Six thousand ninety-nine ambulatory Chinese women living in Hong Kong and 6302 mainland Chinese women living in Beijing were recruited for the study. The BMD was found to be 2.2% higher at the lumbar spine, 8.4% higher at the total hip, and 7.2% higher at the femoral neck in Beijing women than those in Hong Kong women. However, after adjustment for age, weight, and height, this trend was reversed so that the adjusted BMD at the lumbar spine, total hip, and femoral neck was 4.8%, 2.7%, and 1.4% higher in Hong Kong Chinese women than Beijing Chinese women, respectively. Body weight accounted for 13.3%, 14.6%, and 10.6% of the difference in BMD of lumbar spine, total hip, and femoral neck between the 2 populations. The prevalence of osteoporosis in Hong Kong women (24.9%) was found to be higher than that in Beijing women (20.3%). We conclude that osteoporosis is a major health problem in Chinese women, and in comparing BMD between subjects of the same ethnicity, body weight must be taken into account.