We studied the effect of pubertal development on insulin secretion. Intravenous glucose tolerance tests were performed on 47 islet-cell antibody negative siblings of diabetic children and on 16 normal adults. Puberty was staged using Tanner's criteria and subjects were grouped as follows: I, stage 1 (n = 16); II, stages 2 and 3 (n = 15); III, stages 4 and 5 (n = 16); IV, adults (n = 16). Fasting insulin increased with advancing puberty (p = 0.59, P less than 0.001). The stimulated insulin response also rose with increasing pubertal development: for the 0-10 min insulin area, p = 0.46, P less than 0.001 and for the 10-60 min area, p = 0.68, P less than 0.001. There was a low positive correlation between insulin and age to 16 years but multiple regression analysis showed that this could be accounted for by puberty alone. Indeed prepubertal children and adults did not differ. There was no correlation between glucose (fasting and 0-60 min area) and puberty or age. These findings suggest that insulin resistance increases during puberty, and this may contribute to the frequency of presentation or worsening control of insulin dependent diabetes at this time.