Late-onset 3 beta-hydroxysteroid dehydrogenase (HSD) deficiency was diagnosed in a 30-year-old woman with hirsutism and normal menstrual cycles. No genital abnormalities were present. Elevated basal serum levels of delta 5-3 beta-hydroxysteroids were demonstrated. Serum pregnenolone (P5) was 3.0 ng/ml, and dehydroepiandrosterone sulphate (DHEA-S) 3245 ng/ml. Basal serum levels of delta 4 steroids were low or within normal limits. Serum progesterone (P) was 0.5 ng/ml, 17 alpha-hydroxyprogesterone (17-OHP) 0.2 ng/ml, androstenedione (delta 4A) 0.4 ng/ml, and testosterone (T) 0.1 ng/ml. All delta 5/delta 4 steroid ratios were elevated. Dexamethasone (DEX) administration normalized the elevated levels of delta 5-3 beta-hydroxysteroids, whereas delta 4-3-ketometabolites exhibited only minor modifications. The DHEA-S/delta 4A ratio increased more than five times over the basal ratio, and P5/P and 17 alpha-hydroxypregnenolone (17-OHP5)/17-OHP ratios did not increase after adrenocorticotrophic hormone ACTH stimulation. Studies of basal ovarian function revealed 17 beta-estradiol (E2) and gonadotropins within normal limits according to the menstrual cycle. In the follicular phase, follicle-stimulating hormone (FSH) was 101.3 ng/ml, luteinizing hormone (LH) 46.0 ng/ml, and E2 49.7 pg/ml; in the luteal phase, FSH was 180.0 ng/ml, LH 69.3 ng/ml, and E2 50.1 pg/ml. The presence of ovulatory cycles was documented on the basis of the biphasic pattern of the basal body temperature cycles and the increment in P levels. This case demonstrates the existence of normal ovulatory function in a woman with late-onset of a mild form of HSD.