Insulin-dependent (type 1) diabetics, aged 14-17 years, were studied according to two protocols. During a 6-month training period of moderate intensity (six participants) the aerobic work capacity and the erythrocyte insulin binding increased by 19% and 28%, respectively. Glycosylated haemoglobin (HbA1) was not significantly reduced. A 2-week intensive physical training program (10 participants) was associated with a 50% decrease of blood glucose values, which did not last beyond the training period. Plasma ketone bodies were markedly reduced. We conclude that young type 1 diabetics may participate in strenuous, short-term physical training. The improved aerobic work capacity and increased cellular insulin binding observed during training of moderate intensity is of potential benefit in the long-term management of the patients.