It is now apparent that many cases of amenorrhea. oligomenorrhea. corpus luteum deficiency, galactorrhea, and infertility are due to hyperprolactinemia. We investigated the relationships between serum prolactin values and factors such as menstrual pattern, frequency of galactorrhea etc, in 135 hyperproIactinemic patients at the Seoul National University Hospital during a period of 6 years, from January, 1979 to December, 1984. The results was as follows: 1. Menstrual pattern changed according to the serum prolactin level. The frequency of amenorrhea is 1.7 percent in patients with serum prolactin levels ranged from $25{sim}40ng/ml$, whereas 72.4 percent in patients with serum prolactin levels above 100ng/ml. 2. The incidence of galactorrhea in hyperprolactinemic patients was 3.1 percent and the frequency of galactorrhea had direct relationship with the serum prolactin level and/or the frequency of abnormal menstrual pattern. 3. The incidence of pituitary tumor in hyperprolactinemic patients was 104 percent and sixty percent of patients with serum prolactin levels above 100ng/ml had a pituitary tumor . 4. There was an inverse correlation between serum prolactin and progesletone value. 5. The frequency of anovulatory menstrual cycle evidenced by basal body temperature is 23.9 percent in patients with serum prolactin levels ranged from $20{sim}40ng/ml$, whereas 76.9 percent in patients with serum prolactin levels above 100ng/ml.