Thirty-one patients who became pregnant after in vitro fertilization were compared with a matched control group of 31 patients who underwent embryo transfer but did not conceive. During the follicular phase there was no difference in the mean daily estradiol (E2) levels between the pregnant and control groups. In 95% of the women who conceived, E2 continued to rise or plateau on the day after human chorionic gonadotropin (hCG) administration; this pattern was seen in only 62.5% of the control group. There were significantly more 'regular' embryos (i.e., those with symmetric blastomeres and no visible cytoplasmic fragments) replaced in patients who became pregnant, compared with the control subjects (65.6% versus 47.0%, P < 0.05). Embryo regularity was associated with preovulatory E2 pattern; falling E2 levels on the day after hCG administration were associated with transfer of only 18.8% of regular embryos in the control group. Whereas in the conceptional cycles with rising E2 levels, 61.5% of the replaced embryos were morphologically regular (P < 0.01). These results would indicate that changes in the follicular environment as reflected by peripheral E2 levels might affect the development potential of the human embryo.