Fertility was evaluated in 53 female patients with late-onset adrenal hyperplasia (LAH) due to 21-hydroxylase deficiency. The majority of patients (n = 33) were seen for isolated postpubertal hirsutism, 9 patients consulted for sterility, and 11 for irregular menstrual cycles. At the time of diagnosis, the ages of patients ranged from 15-40 yr (mean +/- SD, 24.6 +/- 5.2). No patient had major signs of virilization. The plasma 17-hydroxyprogesterone level was higher than normal in all patients (26.8 +/- 18.9 nmol/L; range, 3.4-139.4) and dramatically increased to 140.1 +/- 80.6 nmol/L (range, 35.2-324.2) after ACTH treatment. Plasma androgen levels were high (testosterone, 3.25 +/- 2.03 nmol/L; delta 4-androstenedione, 13.65 +/- 5.60 nmol/L). Plasma basal and LHRH-stimulated values were normal for FSH and high for LH. Basal and TRH-stimulated plasma PRL levels were normal. Among these 53 LAH patients, only 20 desired a pregnancy. These had a total of 38 pregnancies. Ten patients became pregnant before the diagnosis of LAH and without any treatment; they had a total of 18 pregnancies, 12 of which were successful. Moreover, 19 normal pregnancies without any spontaneous abortion were carried to term by 14 of 16 hydrocortisone-treated patients. One patient needed the association of one cure of clomiphene citrate. Hypofertility in LAH patients seems, therefore, to be relative. Its mechanism is hormonal, with anovulation or dysovulation, due to the continuous steroid feedback of adrenal origin on the hypothalamo-pituitary axis. Hydrocortisone is the appropriate treatment in most cases, reducing adrenal androgen overproduction and relieving hypothalamic-pituitary gonadotropin function, thereby making possible cyclic ovarian activity and ovulations.