Excessive GH response to various stimuli has been frequently described in diabetes mellitus. We studied the GH response to a synthetic GHRH in a group of 16 non-obese Type I diabetic patients. GHRH (1-44) given as iv bolus at a dose of 50 micrograms induced a markedly greater GH response in the diabetic than in the normal subjects with peak values of 39.5 and 14.7 micrograms/l, respectively, and the differences were significant from 15 to 60 min. Peak GH level was 44.6 micrograms/l in diabetic patients without retinopathy and 34.2 micrograms/l in patients with retinopathy, but the difference was not significant. Peak GH levels did not correlate with metabolic control of disease estimated by basal glucose and HbA1 levels nor with age, weight of the patients, and duration of the disease. It is concluded that Type I diabetic patients show an exaggerated GH response to GHRH and this response does not correlate with the metabolic control of diabetes.