Previous reports have suggested a predilection for right-sided ovulation and that epithelial ovarian cancer arises more commonly in the right ovary, thus establishing a possible causal relationship between ovulation and ovarian cancer. However, evaluation of ovulation patterns in consecutive cycles has produced contradictory results and there are no reports corroborating a higher incidence of right-sided ovarian cancers. We reviewed the records of 293 patients with epithelial ovarian cancer and found no significant difference between the frequency of tumors in the right and left ovaries. We also prospectively analyzed the side of ovulation in 41 unselected consecutive patients undergoing 156 in vitro fertilization cycles without ovarian stimulation. Follicular development was carefully monitored by daily ultrasonic scan and estradiol and LH plasma determinations. Eight-two ovulations (52%) were on the right (not statistically significant). Thus our results bring into question previous studies suggesting a link between the side of ovulation and epithelial ovarian cancer.