The aim of the study was to evaluate a possible lateral difference in ovarian activity and its effect on cycle fecundity. A database was analysed retrospectively which covered 477 cycles in which frozen/thawed embryo transfer had been carried out. The cycles were spontaneous, with no hormonal treatment. Women with ovulation problems as a reason for infertility treatment were excluded. Factors investigated were the side of ovulation, endometrial thickness on cycle days 10-12 and on the day of embryo transfer, and pregnancy rate per embryo transfer. Ovulation was right-sided in 273 of the 477 cycles (57.2%) and left-sided in 204 of the cycles (42. 8%) (95% CI 38.3-47.2, P = 0.002). In the age category of 30-37 years, covering 288 cycles, the incidence of left-sided ovulation was 126 (43.7%, 95% CI 38.0-49.5, P = 0.034). In this category, the endometrial thickness (+/-SD) was significantly greater on the day of embryo transfer (i.e. at time of implantation) when there had been left-sided ovulation compared with right-sided [9.6 mm (2.0) versus 9.1 mm (1.8), P = 0.037]. In addition, the pregnancy rate per embryo transfer was higher when ovulation had been on the left side [32/126 (25.2%) versus right side 24/162 (14.8%), P = 0.035, 95% CI 0.0122-0.199]. In conclusion, right-sided ovulation was more frequent than left-sided in the whole group. This is the first study to report that the side of ovulation has a clinical impact. These data support the hypothesis that the side of ovulation is significant in terms of embryo implantation.