OBJECTIVE: The aim of this study was to establish whether type 1 diabetes has a long-term effect on bone development in children and adolescents. RESEARCH DESIGN AND METHODS: Bone characteristics and muscle cross-sectional area (CSA) were analyzed cross-sectionally in 41 (19 female and 22 male) patients and were reevaluated after 5.56 +/- 0.4 years using peripheral quantitative computed tomography (pQCT). We hypothesize that bone size and muscle mass normalize with age. RESULTS: At the first evaluation, mean +/- SD age was 9.87 +/- 2.3 years and disease duration was 4.31 +/- 2.9 years. Height was -0.36 +/- 1.9 SD, and BMI was 0.39 +/- 0.9 SD. Parameters of bone size were low in the whole patient group (corrected for patient's height). At reevaluation, age was 15.44 +/- 2.3 years, and patients had a mean height of -0.12 +/- 0.8 SD. BMI SD had increased to 0.57 +/- 1.1. Total and cortical CSA had normalized. Those patients with an increase in total CSA had a significant younger age at disease manifestation and a younger age at initial pQCT measurement. Bone size was well adapted to muscle mass expressed as the ratio of bone mineral content per muscle mass, and a close correlation was shown between the increase in bone size and in muscle CSA (r = 0.46, P = 0.03). CONCLUSIONS: Patients with manifestation of type 1 diabetes at an early age had transient impaired bone development. Within the follow-up period, the greatest increase in bone size was found in these patients. In adolescence, all patients had a normal bone size and appropriate adaptation of bone on muscle.