The object of the study was, first, to investigate whether girls suffering from insulin-dependent diabetes mellitus (IDDM) are more overweight than an age- and puberty-matched control group and, second, to study the impact of diet, calorie intake and pubertal stage on body mass index (BMI), body weight and fat content. We studied 43 girls with IDDM and controls, divided into two age groups: group 1 (n = 21; 10-13 years) and group 2 (n = 22; > 13 years, 13.1-20.7 years). Overweight was assessed by BMI, relative weight and body fat from skinfold thickness. Food consumption data were collected over a one week food and drink protocol. The diabetic girls, particularly those after puberty, were more overweight than the controls. Although the calorie intake was increased compared with their peers, the proportions of energy derived from protein, fat and carbohydrate were as recommended by the American and Swiss Diabetes Association. Most importantly, the recommended proportion of saturated fatty acids (< 10%) was not achieved by either the diabetic patients or the control girls. Insulin dose/unit body weight correlated with BMI and fat content. Therefore, the increased insulin dose may be responsible for the relatively increased energy intake and, in addition, increased intake of saturated fatty acid which has been related to poor metabolic control and obesity. The food intake of the control girls was identical to that reported in adults by the Swiss Government in 1991 in the Third Report on Food Consumption.