Monitoring of human menopausal gonadotropin (hMG) treatment for induction of ovulation according to either preovulatory estrogen levels or the presence of a dominant ovarian follicle was found insufficient to prevent ovarian hyperstimulation syndrome (OHS). In 65 infertile patients treated with hMG and human chorionic gonadotropin (hCG), a possible correlation between the number and size of all ovarian follicles on the day of assumed ovulation and the occurrence of OHS was evaluated in order to assess the value of ultrasonography in predicting OHS. It was found that patients with OHS had significantly more follicles at the time of hCG than patients without OHS. Mild OHS was characterized by the presence of eight to nine follicles, 68.7% of which were of intermediate size (9 to 15 mm). In moderate to severe OHS 95% of the preovulatory follicles were <16mm, most of them (54.7%) <9 mm in diameter. It can be concluded that a specific preovulatory follicular configuration characterizes mild and severe hyperstimulation. This is important information before hCG administration and emphasizes the value of ovarian ultrasonography in predicting OHS.