The authors planned to show the effect of high-level estrogen on cognitive functions, using P300 in the in-vitro fertilization (IVF) population. Eighteen IVF patients (mean age 31.39 +/- 5.01) without any detectable cognitive disorder were enrolled in this study. The authors measured N100, P200, N200, P300 latencies and amplitudes at Cz and Fz recordings during the lowest and highest blood estradiol levels. P300 amplitudes were significantly reduced during the highest level of estradiol compared to the lowest level both at Fz and Cz recordings. The other subcomponents of P300 were not significantly influenced by blood estradiol levels. The results revealed a decline in the ability of task relevance and state of arousal when the estradiol level was the highest in the IVF patients.