Pregnancy outcomes were evaluated retrospectively in 350 women to investigate the relationship between endometriosis and spontaneous abortions. The frequency of spontaneous abortions in women with endometriosis was significantly higher than in both a fertile nonendometriosis group and an infertile group with tubal disease. There was no correlation between the severity of the endometriosis and the frequency of spontaneous abortions. After treatment, the frequency of spontaneous abortions was significantly decreased in both the endometriosis and the tubal disease group, but these values were not significantly different from each other. We conclude that high spontaneous abortion rates are a characteristic of other subgroups of women with secondary infertility and not just in women with endometriosis, and that the majority of spontaneous abortions associated with endometriosis are not caused by the condition.