Severe virilization developed in a pregnant woman in association with the occurrence of theca lutein cysts and hyperreactio luteinalis of the ovaries. At term, maternal serum levels of androstenedione (58 ng/mL) and testosterone (20 ng/mL) were elevated massively; estrone (24 ng/mL) and estradiol-17 beta (23 ng/mL) levels were increased moderately. Maternal serum levels of human chorionic gonadotropin (hCG) just before delivery, 22,276 mlU/mL, though twice the mean for normal women at term, were within the normal range. The levels of androstenedione (1.06 ng/mL) and testosterone (0.26 ng/mL) in umbilical cord serum of her normal newborn female infant were normal. The levels of androstenedione and testosterone in serum of this woman declined slowly, but progressively, during the first two months after delivery; the serum levels of androstenedione and testosterone increased substantially, however, after she was treated with hCG eight weeks postpartum. These results are suggestive that, for reasons unknown, markedly increased androgen production with theca lutein cysts and hyperreactio luteinalis in some pregnant women results ultimately because of increased ovarian sensitivity to hCG.