Continuous subcutaneous insulin infusion (CSII) has become increasingly popular as a form of intensified insulin therapy in adolescents with type 1 diabetes mellitus (DM). One reported drawback was increased weight gain in adolescents after initiation of insulin pump therapy. In a prospective, longitudinal, non-randomized and case controlled study, we followed 12 adolescents (mean age 13.6 yr, 8 males, 4 females) from 6 months before the start of CSII to 12 months on CSII. These 12 adolescents with DM on CSII were matched for age, gender, HbA1c, duration of DM, and body mass index (BMI) with 12 adolescents who continued on multiple injection therapy (MIT). In addition, six of the 12 adolescents on CSII intended to control their weight by means of the insulin pump. These six vs six adolescents within the CSII group were further analyzed for weight development and eating habits. Clinical indications for CSII were dawn phenomenon, night-time hypoglycemia and patient request for more flexibility in DM management. All patients had been in satisfactory metabolic control on MIT. After 12 months of CSII, the daily insulin requirement remained significantly lower than 18 months before (0.79 +/- 0.11 vs 1.02 +/- 27 U/kg/d, p = 0.034) and number of daily meals was lower (4.1 +/- 0.9 vs 6.5 +/- 0.7, p = 0.006). Mean initial HbA1c was 7.4% in the MIT and CSII patients, and remained comparable between these two groups. BMI was not different between the CSII and MIT group over the entire study period. However, those adolescents on CSII who intended to control their weight by means of the insulin pump were able to achieve relative weight loss during the,first 6 months on CSII. Two patients of the CSII group had one severe hypoglycemic episode with loss of consciousness. In conclusion, CSII does not lead to weight gain by itself, but allows sufficient weight control without a negative effect on metabolic control. The general threat of weight gain in patients who switch to insulin pump therapy must be pointed out, and the role of eating habits and caloric content of food should play a central role in insulin pump educational programs.