Objective: To predict the risk of premature ovulation and possibility of pregnancy in natural IVF/ICSI cycles on the basis of the estradiol (E 2 ) level on the day of hCG injection and on the day after.Design: A prospective study.Setting: Hospital research program.Patient(s): One hundred sixty-four women undergoing 305 IVF/ICSI natural cycles.Intervention(s): Serum E 2 levels were measured ~12 h before (day 0) and 12-17 h after hCG administration (day 1).Main Outcome Measure(s): E 2 levels on day 0 and day 1, the ratio of the day 1 to day 0 levels, and cycle outcome.Result(s): In cycles with premature ovulation and in conception cycles, the average E 2 level on day 0 was statistically significantly higher than in other cycles, whereas the E 2 ratio was statistically significantly lower. Multiple logistic regression was used to determine the connection of the E 2 level on day 0 (B0 = -0.742, B = 2.147, P = .01) and the E 2 ratio (B0 = -0.742, B = -3.135, P<.001) with premature ovulation. Only the E 2 ratio (B0 = 0.659, B = -2.209, P = .0068) was significantly connected with conception.Conclusion(s): In predicting the outcome of natural IVF/ICSI cycles, the importance lies not in the E 2 level on the day of hCG administration or on the day after, but rather in the E 2 ratio. (Fertil Steril ( R) 2001; 75:000-00. (C) by American Society for Reproductive Medicine.)