Fractures are more prevalent among people with severe and profound developmental disabilities than in the general population. In order to characterize the tendency of these people to fracture, and to identify features that may guide the development of preventive strategies, we analyzed fracture epidemiology in people with severe and profound developmental disabilities who lived in a stable environment. Data from a 23-year longitudinal cohort registry of 1434 people with severe and profound developmental disabilities were analyzed to determine the effects of age, gender, mobility, bone fractured, month of fracture, and fracture history upon fracture rates. Eighty-five percent of all fractures involved the extremities. The overall fracture rate increased as mobility increased. In contrast, femoral shaft fracture risk was substantially higher in the least mobile [relative risk (RR), 10.36; 95% confidence interval (CI), 3.29-32.66] compared with the most mobile group. Although the overall fracture rate was not associated with age, the femoral shaft fractures decreased but hand/foot fractures increased with age. Overall fracture risk declined in August and September (RR, 0.70; 95% CI, 0.55-0.89), being especially prominent for tibial/fibular fractures (RR, 0.31; 95% CI, 0.13-0.70). Gender was not a factor in fracture risk. Two primary fracture mechanisms are apparent: one, largely associated with lack of weight-bearing in people with the least mobility, is exemplified by femoral fractures during non-traumatic events as simple as diapering or transfers; the other, probably due to movement- or fall-related trauma, is exemplified by hand/foot fractures in people who ambulate. The fracture experience of people with severe and profound developmental disabilities is unique and, because it differs qualitatively from postmenopausal osteoporosis, may require population-specific methods for assessing risk, for improving bone integrity, and for reduction of falls and accidents.