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Body Composition in Children and Adolescents with Osteogenesis Imperfecta. OBJECTIVE: To use peripheral quantitative computed tomography to determine the cross-sectional area (CSA) of subcutaneous fat and muscle (fat CSA, muscle CSA) in transverse forearm scans in patients with osteogenesis imperfecta (OI). STUDY DESIGN: Fat and muscle CSA were quantified in 266 individuals (142 female) aged 5-20 years who had a diagnosis of OI type I, III, or IV and who had mutations in COL1A1 or COL1A2. Results were compared with those of 255 healthy controls. RESULTS: In a subgroup of 39 patients with OI type I, % fat CSA correlated closely with total body percentage fat mass as determined by dual-energy x-ray absorptiometry (R(2) = 0.69; P < .001). In the entire study cohort, muscle CSA adjusted for age, sex, and forearm length was lower in OI type I and III than in controls (P < .05 each), but fat CSA was similar between OI types and controls. No relationship between the type of disease-causing mutation in the COL1A1 or COL1A2 genes and fat CSA or muscle CSA was found. CONCLUSIONS: Children and adolescents with OI have low muscle size but a normal amount of subcutaneous fat at the forearm.