Our purpose was to assess factors that are associated with an increased rate of spontaneous abortion in pregnancies initiated by in vitro fertilization. Pregnancies were diagnosed by measurement of serum human chorionic gonadotropin (hCG) 15 days after embryo transfer. Of the 64 women who conceived, 47 delivered term infants, one patient delivered a stillborn at 22 weeks, 14 aborted in the first trimester, and two had pregnancies that implanted in the tube. Abortion rates were similar for women treated with human menopausal gonadotropin (24%; 12 of 54) and those who received clomiphene citrate (12.5%; one of eight). Two patients conceived after treatment with a combination of clomiphene citrate and human menopausal gonadotropin, neither of whom aborted. In 54 patients treated with human menopausal gonadotropin, there were no significant differences in mean maternal age, number of years of infertility before the pregnancy, history of previous pregnancies, amount of human menopausal gonadotropin used to induce ovulation, serum estradiol levels on the day of hCG administration, mean number of follicles, and the mean number of transferred embryos between the group who delivered and the group who aborted. We conclude that none of these factors are associated with increased tendency for fetal loss in our in vitro fertilization program. Beta-hCG levels on day 15 after embryo transfer were significantly lower in the group who aborted than in the group who delivered, and may be predictive of implantation failure.