The mechanism of infertility associated with endometriosis is poorly understood. There is evidence supporting that women with ovarian endometriosis have a lower pregnancy rate than women with peritoneal lesions only. This study aimed to evaluate the effects of endometriotic contents contamination while retrieving oocytes on oocytes' quality. Thirty-eight infertile patients with endometriotic cysts from January 1993 to June 2000 were enrolled in this study. There were no statistically significant differences among the quality of oocytes and embryos from the contaminated, non-contaminated, and control group. However, the fertilization rate and pregnancy rate were impaired by the contamination of endometriotic contents. We conclude that ovarian endometriosis should be treated before starting in vitro fertilization program in order to increase the pregnancy outcome.