OBJECTIVE: To examine the differential effects of intensive and conventional diabetes therapy on weight gain and body composition in adults with type 1 diabetes. RESEARCH DESIGN AND METHODS: Between 1982 and 1989, 1,246 adults (aged 18-39 years) in the Diabetes Control and Complications Trial were randomly assigned to either conventional therapy (1-2 injections of insulin per day) or intensive therapy (multiple daily injections or continuous subcutaneous infusion with frequent blood-glucose testing). Height and weight were measured at baseline and at annual visits for an average of 6 years (range 3-9). Body composition was assessed cross-sectionally with bioelectrical impedance analysis during 1992, at which time waist and hip circumferences were measured. RESULTS: Intensively treated patients gained an average of 4.75 kg more than their conventionally treated counterparts (P < 0.0001). This represented excess increases in BMI of 1.5 kg/m(2) among men and 1.8 kg/m(2) among women. Growth-curve analysis showed that weight gain was most rapid during the first year of therapy. Intensive therapy patients were also more likely to become overweight (BMI >or=27.8 kg/m(2) for men, >or=27.3 kg/m(2) for women) or experience major weight gain (BMI increased >or=5 kg/m(2)). Waist-to-hip ratios, however, did not differ between treatment groups. Major weight gain was associated with higher percentages of body fat and greater fat-free mass, but among patients without major weight gain, those receiving intensive therapy had greater fat-free mass with no difference in adiposity. CONCLUSIONS: Intensive therapy for type 1 diabetes produces substantial excess weight gain compared with conventional therapy. However, the additional weight appears to include lean tissue as well as fat.