Endocrine function was studied in 12 patients, 7 men and 5 women, with myotonic dystrophy (MD). Growth hormone (GH) was within normal limits in all the patients and there was no response to arginine stimulation. Prolactin (PRL) was above normal in only 2 cases (one man and one woman) and the response to TRH was below normal in 2 patients and slightly above in one. The GnRH test yielded a reduced LH response in 4 of the 7 men and none at all in the 4 women in whom it was done. The FSH response was below normal in only one of the 7 men and in 3 out of 4 women. The testosterone assay after HCG stimulation was borderline high in 2 out of 6 men and below normal in one. The level of thyroid hormones (T3, T4, FTI, TSH) was normal in all patients except one, whose FTI and T4 level were below normal. In the TRH test TSH was raised in only one of the 6 patients tested. The circadian rhythm of cortisol was absent in 3 out of 10 patients and 3 out of 8 patients showed no response to ACTH stimulation. The results of the study suggest that endocrine alterations are fairly frequent in MD but that they are neither specific nor correlated with disease severity or duration.