Sixteen patients with polycystic ovarian disease (PCOD) were treated for 39 cycles with pure follicle-stimulating hormone (FSH) for the induction of ovulation. At ovulation time human chorionic gonadotropin (hCG) was administered. Twenty-one cycles were ovulatory. Twenty-three were classified as normostimulated (N): six pregnancies and three abortions were observed. In the remaining eight hyperstimulated (H) cycles there were four full-term pregnancies. Dosage and length of treatment were greater in patients with excess body weight (P less than 0.01). H cycles were characterized in respect to N cycles by: (1) higher baseline values of 17-hydroxy progesterone (17-OHP) plasma levels and LH/FSH ratios; (2) higher plasma concentrations and rate of increase of 17-OHP periovulatory levels. E2 plasma levels did not permit a clear differentiation between H and N cycles, and it was not useful for the timely recognition of hyperstimulation. Our data show that a slight controlled degree of ovarian hyperstimulation is beneficial to pregnancy rate and outcome.