OBJECTIVE To determine if the dose of hCG affects the initial rise in progesterone seen in patients undergoing IVF-ET and therefore affects it usefulness as a predictor of cycle outcome. DESIGN Comparison of the rise in progesterone with cycle outcomes for IVF patients receiving 5,000 or 10,000 mIU hCG to stimulate oocyte maturation. SETTING University-based infertility program. Patients-One hundred six patients undergoing IVF-ET on a long protocol of down-regulation with GnRH, hMG stimulation, and hCG to stimulate oocyte maturation. Stimulation protocol varied only in dose of hCG [5,000 mIU (N = 72) vs. 10,000 mIU (N = 34)]. MAIN OUTCOME MEASURE(S) Rise in progesterone from 12 hours before to 12 hours after hCG administration and its relationship with cycle outcome. RESULTS All 106 women exhibited a rise in progesterone following the administration of hCG. As seen in earlier studies, there appeared to be a relationship between minimal progesterone increases (<3-fold) and cycle failure in patients receiving 5,000 mIU (P < .02). However, using the criteria of the previous study, there appears to be no relationship between progesterone and cycle outcome in patients receiving 10,000 mIU (P = .30). Further, the higher dose of hCG appeared to induce greater increases in progesterone over the 24-hour period examined (P < .02). After readjustment of the critical value to 3.5-fold, there was an increased tendency toward cycle failure in women exhibiting a minimal progesterone increase. Unlike the relationship associated with 5,000 mIU hCG, though, the relationship between 10,000 mIU hCG and progesterone levels was not statistically different (P = .10). CONCLUSIONS Increasing the dose of hCG used to stimulate oocyte maturity shifts the previously described relationship between progesterone and IVF-ET-cycle outcome. However, while it remains unclear if progesterone can be used as a predictor of outcome at the higher hCG dose, it appears clear that a relationship exists between minimal progesterone response to hCG and cycle failure.