Background: Insulin is an important osteotropic hormone, but may be negatively associated with bone mass after adjustment for body mass index in adolescent populations. However, the association between insulin and bone mass in adults remains unclear. Objective: To investigate whether insulin was associated with bone mass in adults, and if so, whether the association was positive or negative. Design: Cross-sectional, using data from the Fourth Korea National Health and Nutrition Examination Survey (KNHANES) 2008-2009 and the U.S. National Health and Nutrition Examination Survey (NHANES) 1999-2006. Setting: Korean and U.S. Population Participants: 7271 KNHANES and 3399 NHANES participants were included. Main Outcome Measures: Anthropometric parameters and bone mass data, fasting glucose and insulin, height, weight, and markers related to insulin resistance. Results: After adjusting for confounding factors, there was an inverse relationship between insulin and total body bone mineral content (BMC) in the KNHANES and NHANES subjects. In a stratified analysis, an association between insulin and bone mass was apparent, especially in the highest homeostatic model of assessment of insulin resistance (HOMA-IR) quartile in the Korean subjects. However, this association was only seen in men in the U.S. subjects. Conclusions: There is an inverse relationship between insulin and total body BMC after adjustment for confounding factors in Korean and U.S. subjects, especially in the insulin-resistant population. This strongly suggests that the adverse influence of insulin on bone mass likely reflects the effects of other factors associated with insulin resistance rather than being a direct action of insulin itself.