The therapeutic effectiveness of bromocriptine was studied in 24 patients (40 cycles) with luteal insufficiency accompanied with hyperprolactinemia in the follicular phase. Treatment with bromocriptine was started on day 5 of each menstrual cycle and continued for 7 days per cycle. In the first menstrual cycle, this 7-day treatment lowered the blood prolactin level, and significantly increased blood estradiol and progesterone in 83.3% of the patients. Among the patients whose prolactin levels were not lowered by bromocriptine, only a few showed increases in estradiol or progesterone. In many of the patients whose prolactin levels were excessively lowered to 5 ng/ml or less, progesterone was decreased. These results suggests that bromocriptine produces a favorable effect on hyperprolactinemic luteal insufficiency by secondarily stimulating progesterone secretion as a result of suppressing the follicular growth "inhibiting action of prolactin. Prolactin of an appropriate concentration, however, was considered to be needed for the maintenance of normal function of the corpus luteum.