Growth hormone (GH) levels were measured in 12 patients with myotonic dystrophy (MD; 7 men and 5 women, aged 21-49 years) and 14 volunteers after administration of 100 micrograms GH-releasing hormone (GHRH; 1-29). A 75-g oral glucose tolerance test was carried out to determine glucose, insulin, plasma C-peptide, and urinary C-peptide. The GH level in six MD patients responded normally to GHRH (group I), with a peak of 17.1 +/- 1.46 micrograms/l, compared with controls (27.8 +/- 19.6 micrograms/l, NS), and that in the other six patients responded subnormally, with a peak of 3.15 +/- 1.46 micrograms/l, lower than in controls and in group I patients (P < 0.001). In group I the insulin response to the glucose tolerance test showed hyperinsulinism and was lower than that in group II patients; stimulated C-peptide was also higher in group II than in group I and in controls; urinary C-peptide levels were parallel to those in previous data. In all MD patients there were a negative correlation between absolute values of GH response to GHRH and insulin response to glucose tolerance test (r = -0.79, P < 0.001). Our data suggest that the failure in GH release and peripheral insulin action is due to a generalized defect in cellular membrane function in MD patients.