Hypothalamic-pituitary-testicular function was studied in 70 patients with myotonic dystrophy (MD). The diagnosis was confirmed by electromyography. The mean age of the patients was 36.2 +/- 13.2 yr and the duration of the disease was 11.17 +/- 8.01 yr. Testicular atrophy (testes less than or equal to 12 ml on a Prader orchidometer) was present in 65.5% of patients. Fertility among married patients was 66.6%. Mean testosterone plasma levels were 438 +/- 298 ng/dl vs 520 +/- 185 ng/dl in the control group (P = NS). Basal plasma FSH and LH levels, and their response after the administration of 100 mcg of LH-RH were significantly increased although a wide dispersion was observed. Sperm count was carried out in 27 cases, showing a normal count in 7, oligospermia in 12, and azoospermia in 8 patients. Testicular biopsy was performed in 45 patients being normal in 2, showing mild testicular damage in 8, moderate in 14, and severe in 18; it was nule in 3 of them. A significant relationship between testicular atrophy and the sperm count (p less than 0.01), testicular damage and testicular atrophy (p less than 0.025), and sperm count and testicular damage (p = 0.017) was found. Basal plasma FSH and LH level were significantly related to the degree of damage in the testicular biopsy. All these findings indicate a primary testicular pathology, prevailing tubular over interstitial damage. We have not found any association between the duration of the disease and gonadal dysfunction.