OBJECTIVE In order to identify parameters which predict prognosis for success with in vitro fertilization, 17-hydroxyprogesterone and progesterone levels were evaluated in 254 patients undergoing 296 in vitro fertilization cycles. Selected response and outcome data were recorded. RESULTS Patients with intermediate values of serum progesterone (0.7-0.8 ng/ml) at the time of human chorionic gonadotropin administration achieved significantly higher pregnancy rates than patients with lower (< 0.7 ng/ml) or higher (> 0.8 ng/ml) levels. The clinical pregnancy rates were 46%, 31%, and 27% respectively (P = 0.02). There was no change in 17-hydroxyprogesterone concentration which predicted a higher pregnancy rate. CONCLUSIONS Excellent clinical pregnancy rates were noted in cycles with a progesterone level of 0.7-0.8 ng/ml, as well as good results in cycles above 0.8 ng/ml. There is therefore no reason to administer human chorionic gonadotropin at a smaller follicle size to prevent a rise in serum progesterone.