To determine the incidence of hyperprolactinemia and prolactinoma without symptoms in men, the serum levels of prolactin were measured in 4803 men. Of these, 14 had hyperprolactinemia with prolactin level of over 50 ng/ml, and 3 had prolactin levels of more than 500 ng/ml. Of these 3 subjects, 2 were found to have pituitary prolactinoma by computed tomograph scanning and surgery, the other subject, who was highly suspected to have prolactinoma, refused further examinations. The serum prolactin levels of 51–100 ng/ml in 11 subjects may have been induced by drugs (sulpiride) or unknown factors. The present survey suggests that the incidence of prolactinoma in men in the general population might be estimated to be 1:1600, and that mass screening is useful for early diagnosis of asymptomatic prolactinoma.