Overweight in insulin-dependent diabetes mellitus (IDDM) has been repeatedly reported, especially in girls during adolescence. Potential pathophysiologic factors include tight metabolic control, insulin dose, treatment regimen, puberty and genetics. A standardized data-base from all IDDM patients treated at our institution was evaluated. IDDM patients with hypothyroidism or celiac's disease as well as all records from the first year of diabetes were excluded, resulting in a total of 427 patients (2454 patient-years) available for analysis. BMI and SD-score for BMI based on the Zurich longitudinal growth study were evaluated. Standardized BMI was higher in pubertal children ( + 1.07+/-0.06) compared to prepubertal children (+ 0.68+/-0.07; p < 0.002). This increase was present both for boys and girls. Increasing overweight during puberty was found irrespective of the age at diagnosis of diabetes (prepubertal or pubertal). The daily dose of insulin and the long-term metabolic control had only a minor impact on the development of overweight. In contrast, in pubertal children, SDS-BMI was significantly higher in patients on intensified insulin regimens (3 or 4 daily injections) compared to patients with 2 injections (p < 0.05). These data demonstrate that both boys as well as girls with IDDM develop overweight during puberty. Multiple injection therapy, not daily dose of insulin or the level of metabolic control achieved, was the main predictor of weight gain. This finding may be explained by increased caloric intake due to the flexibility allowed by intensified treatment.