To determine the extent to which spontaneous plasma GH concentrations are abnormal in adolescents with insulin-dependent diabetes mellitus we performed 12-h overnight plasma GH profiles in 21 diabetic adolescents (11 males and 10 females; aged 9.8-16.5 yr; median, 13.6 yr) and 34 healthy adolescent controls (17 males and 17 females; aged 9.1-20.9 yr; median, 13.1 yr). Data were analyzed using the pulse detection program Pulsar and time series analysis, and are presented with respect to age and puberty stage. Mean and maximum GH concentrations, sum of the peak amplitudes, and mean calculated baseline concentrations in the normal children were higher during puberty; highest levels were seen in girls at puberty stages 2-3, and in boys at stages 4-5. A similar pattern was observed in the diabetic adolescents, but all measures of pulse height and mean calculated baseline concentrations were significantly greater than those in the normal subjects (multivariate analysis, P less than 0.001). Pulse frequency did not change during puberty in the normal or diabetic subjects, and the dominant pulse periodicity in both groups was about 180 min. We conclude that the predominant change in GH release during puberty is in pulse amplitude, and that this is increased in diabetes, whereas pulse frequency remains constant in both normal and diabetic adolescents.