Background: Down syndrome (DS) is a frequent cause of intellectual disability. With the increasing life expectancy of these patients, concerns have been raised about the risk of osteoporosis. In fact, several investigators have reported a reduced bone mass in DS. However, the results may be confounded by comorbid diseases, and differences in lifestyle habits and body size. Therefore, we planned to determine anthropometric and lifestyle factors influencing bone mineral density (BMD) in young adults with DS. Methods: Thirty-nine patients with DS (mean age 26 years) and 78 controls were studied. Areal BMD was measured by dual x-ray decsitometry (DXA); volumetric BMD at the lumbar spine and femoral neck was estimated with published formulae. Results: DS patients had lower areal BMD than controls at all regions (spine, hip and total body). Height and projected bone area were also lower. There were no differences between both groups regarding estimated volumetric BMD at the femoral neck. However, spine volumetric BMD was also lower in DS than controls. In multivariate analysis, DS, male sex, little physical activity and low sunlight exposure were associated with lower spine volumetric BMD; on the other hand, fat mass and sunlight exposure were associated with femoral neck volumetric BMD. Conclusion: This study shows that patients with DS had a reduced areal BMD, but it is in part a consequence of the reduced body size, particularly at the femoral neck. Physical activity and sunlight exposure are associated to volumetric BMD and should be stimulated in order to maintain an adequate bone mass in these patients. (Contains 2 figures and 4 tables.)