OBJECTIVES: To determine the serum prolactin levels in women at sea level and at high altitude, and the prevalence of hyperprolactinemia in both places. METHODS: The study included 303 normal women and 112 women with any reproductive dysfunction (menstrual abnormalities, galactorrhea or dysmenorrhea). From these, 228 were living in Lima at 150 m above sea level and 187 were living in Cerro de Pasco at 4340 m. Normal women included nonpregnant and pregnant women. Menstrual abnormalities included amenorrhea, oligomenorrhea and polymenorrhea. RESULTS: Serum prolactin levels were significantly lower in nonpregnant and pregnant women living at high altitude than at sea level. Prevalence of hyperprolactinemia, galactorrhea and menstrual abnormalities were significantly lower at high altitude than at sea level. CONCLUSIONS: It is concluded that menstrual abnormalities associated with hyperprolactinemia is a rare condition at high altitude. The low serum prolactin level observed at high altitude could be due to a high dopaminergic activity. © 1993.