The importance of monitoring luteinizing hormone (LH) secretion during gonadotropin stimulation remains controversial. In the present study, the authors evaluated the occurrence of spontaneous LH surges in 170 cycles stimulated by clomiphene citrate and human menopausal gonadotropin, and correlated the success rate of embryo cleavage to the time interval between the occurrence of the LH surge peak value and the time of human chorionic gonadotropin (hCG) administration. LH was quantitated from urine by an avidin-biotin enzyme immunoassay. The results indicated that a spontaneous LH surge occurred in 18% of the cycles. The number of oocytes recovered was not affected by the occurrence of a spontaneous LH surge. In 12% of all cases, the spontaneous LH surge occurred less than 12 hours before the administration of hCG, and in these cases embryo cleavage was not reduced. In 6% of all cases, the spontaneous LH surge occurred over 12 hours before hCG administration, and in these cases embryo cleavage was reduced significantly.