Transient hyperprolactinemia is associated with a midcycle luteinizing hormone surge. This study demonstrates that LH surge cycles in IVF patients were associated with significantly higher serum PRL concentrations than cycles in which a spontaneous LH surge did not occur. Our findings support the hypothesis of concomitant LH and PRL release at the time of the midcycle gonadotropin surge, as well as that of estrogen sensitization of pituitary lactotropes, and suggest a possible mechanism for transient midcycle hyperprolactinemia. 