Identification of the ovary from which ovulation takes place in two consecutive menstrual cycles results in one of four ovulation patterns, contralateral and ipsilateral ovulations on the right or on the left ovary. Our data suggest that IVF and IUI treatment in cycles in which development of the dominant follicle occurs in the right-sided ovary--and ovulation took place from the left-sided ovary in the preceding cycle (contralateral right-sided ovulation)--is likely to show the best pregnancy outcome.