AIM : Diabetic patients, particularly girls, often experience poor metabolic control during puberty and adolescence. The aim of this study was to investigate metabolic control during adolescence, especially in relation to pubertal stages, growth, insulin treatment and body mass index (BMI). METHODS : We studied the records of 38 (consecutive) girls with prepubertal onset of Type 1 diabetes mellitus. Data from the age of 10 to 18-20 years were obtained with regard to glycaemic control, growth, age at menarche, final height and BMI, and analysed in relation to both chronological age and age at menarche. RESULTS : HbA1c was lowest 3 years before menarche; mean (+/- sd) 7.6 (+/- 1.2). After the pubertal growth spurt, there was a marked impairment of metabolic control, the highest level of HbA1c occurring 3 years after menarche. Mean age at menarche was 13.3 (+/- 1.1) years and mean linear growth after menarche only 4.7 cm, giving a final height of 164.9 (+/- 5.3) cm which is 2.7 cm below the Swedish mean. During adolescence the degree of correlation between BMI and HbA1c continuously increased, pointing out the effect of body fat on metabolic control in this age group. The level of HbA1c at 10 years of age could not predict the metabolic control after cessation of puberty, but prepubertal BMI appears to be a risk factor for both obesity and poor glycaemic control in late adolescence. CONCLUSIONS : The highest HbA1c was found after cessation of growth. Prepubertal BMI is a possible predictor of metabolic control in adolescent diabetic girls.