Ten children with either Sillence type III or IV Osteogenesis Imperfecta have participated in a randomized prospective study evaluating the discontinuation of long leg braces. Each patient was paired with a partner determined to have similar clinical characteristics. Within each of 5 pairs, one child received braces for 16 months and the other did not. Then the assignment was switched. The children were evaluated every four months at NIH. At each admission they received the following: measures of height, weight, bone density (vertebral body, femur, radius), x-rays of long bones of lower extremity and spine, muscle strength testing (MMT) videobased gait analysis, range of motion (ROM). In addition, we have received monthly information completed by parents, describing each child's activities for a two week period. Data analyzed for the first 16 months show 5 fractures occurred in the braced and 7 in the unbraced group. Tibial bowing increased slightly in the unbraced group. Muscle strength in pelvis and leg decreased more in the braced group. Amount of strenuous activity was the same in both groups, but the braced group had more moderate and less minimal activity.