Differences in bone mineral density (BMD) have been observed between adults with Down syndrome (DS) and the general population. The purpose of this article is to describe the prevalence of bone mass disorders in a cohort of adults with DS and their predisposing factors. We performed a cross-sectional study of 104 consecutively recruited adults with DS from an outpatient clinic of a tertiary care hospital in Madrid, Spain. We recorded epidemiological and anthropometric data, nutritional variables, coexisting clinical conditions, and laboratory variables. BMD was measured at the lumbar spine, total hip, and femoral neck using dual-energy X-ray absorptiometry. The prevalence of osteopenia ranged from 48% to 52%, and that of osteoporosis ranged from 19% to 22% depending on the site of measurement (femoral neck or lumbar spine, respectively). Age was the greatest risk factor associated for lower BMD, with similar bone mass accrual curve but with lower peak of BMD than the general population. We conclude that low bone mass is an extremely prevalent condition in adult patients with DS.