OBJECTIVE To investigate whether girls with insulin dependent diabetes mellitus (IDDM) were more overweight than nondiabetic girls, and how diet, insulin treatment, metabolic control, age, and pubertal status were related to body weight and fat content. METHODS Case-control study. METHODS 48 IDDM girls aged 10-19 years and controls matched for age and social class participated in the study. Overweight was assessed by body mass index (BMI), relative weight, and body fat from skinfold thicknesses. Food consumption data were collected by a 48 hour recall method. RESULTS The girls with IDDM were more overweight than control girls according to all measures of obesity (for example, mean BMI 20.3 v 18.9 kg/m2). The daily insulin dose/body weight correlated positively with BMI and per cent body fat. CONCLUSIONS Girls with IDDM are more overweight than their peers, which indicates that a more effective prevention of obesity is needed in the treatment of diabetes.