Prevalence of and risk factors for osteoporosis in a community population of 94 young adults with mental retardation was examined. Results show lower bone mineral density in this group than in an age-matched reference population. Factors associated with low bone mineral density included small body size, hypogonadism, and Down syndrome in both genders and a high phosphate level in females. Low vitamin D levels were common in both genders, despite high levels of exposure to sunshine. A history of fracture was also common. Low bone mineral density and fracture were associated in females but not males. Because morbidity following fracture is likely to be more serious in this population, further investigation of osteoporosis and prevention strategies for both osteoporosis and fractures are important.