The incidence of transient hyperprolactinemia and its impact on in vitro fertilization (IVF) were determined in 151 euprolactinemic women with tubal infertility undergoing an identical gonadotropin stimulation for IVF. Prolactin (PRL) levels were measured on the morning of cycle day 3, days of human chorionic gonadotropin (hCG) administration, and peak estradiol (E2), and in the midluteal phase. Women were divided into high (H: peak E2 greater than 1,000 pg/mL, n = 51), intermediate (I:peak E2: 500 to 800 pg/mL, n = 50), or low (L:peak E2 less than 400 pg/mL, n = 50) E2 response groups. There was no difference in the incidence of hyperprolactinemia on cycle day 3 between the response groups (H:16%, I: 12%, and L:8%). However, high responders had a higher incidence of hyperprolactinemia than intermediate or low responders on all other study days. The incidence of hyperprolactinemia was greater than baseline (cycle day 3) only in the high responders on the day of peak E2. Serum prolactin was strongly correlated with peak E2 (r = 0.41). There were no differences in the number of preovulatory oocytes retrieved or fertilized or the pregnancy rates between hyperprolactinemic and euprolactinemic patients in each response group or when all hyperprolactinemic and euprolactinemic patients, regardless of E2 response, were compared. Transient hyperprolactinemia during gonadotropin stimulation for IVF occurs and correlates with E2 response but has no impact on IVF outcome.