Steroid hormone profiles during luteal phases after in vitro fertilization(IVF) and embryo transfer(ET) have been evaluated in 83 cycles stimulated by pure follicle-stimulating hormone/human menopausal gonadotropin/human chorionic gonadotropin, in which 13 patients became pregnant. Serum estradiol($E_2$) and progesterone($P_4$) levels were determined on days 2, 5, 7 and 9 after laparoscopic follicle aspiration. The follicular $E_2$ peak was slightly higher in pregnancies than in failures. Positive correlations were observed between the follicular $E_2$ peaks and the $P_4$ levels on days 5 and 7 of the luteal phase in pregnancies, but no correlation was found in failures. The $E_2$ and $P_4$ levels on days 5 and 7 of the luteal phase were significantly higher in pregnancies than in failures, but not different on days 2 and 9. Values of the $P_4/E_2$ ratio were similar between the two groups. The luteal phase durations were 12 to 19 days and no correlation was observed between the lengths of luteal phase and the luteal $E_2$ or $P_4$ concentrations. These data suggest that high $P_4$ levels in the mid-luteal phase, which have positive correlations with the follicular $E_2$ peaks, might have a favorable influence on the pregnancy success in human IVF.