The effect of transient hyperprolactinemia and its treatment during cycle stimulation on the endocrine response and fertilization rate of human oocytes was studied. Fifty stimulated cycles were included in the study and divided into three groups: group I consisted of 18 cycles with serum prolactin (PRL) levels less than or equal to 25 ng/ml; group II contained 15 cycles, where patients developed PRL levels greater than 25 ng/ml; group III consisted of 17 cycles, where patients, who already developed hyperprolactinemia in a previous cycle, were treated by 3.75 mg bromocriptine daily. The serum estradiol (E2), progesterone (P) and PRL levels 1, 2, and 3 days before and at oocyte retrieval were evaluated. The E2 decrease at oocyte retrieval was significantly steeper in groups I and III. Follicular luteinization was more effective in groups I and III. The fertilization rate in groups I and III was significantly higher than in group II. High serum PRL levels seem to interfere in follicular and oocyte development. The treatment of transient hyperprolactinemia improved the patients' endocrine response and the fertilization rate of oocytes.