It was reported that most females with prolactinoma, idiopathic hyperprolactinemia, or euprolactinemic galactorrhea were reared either without their father or with an alcoholic, violent father. To gain further insight into this association, a group of sisters of patients with prolactinoma (generally exposed to the same environment as the patients') and a control group were studied. Women with paternal deprivation during childhood differed from the women who had normal childhoods in that they had: 1) higher mean serum prolactin concentration (14.7 versus 9.4 ng/mL; P less than .001); 2) higher incidence of hyperprolactinemia (12 of 50 versus three of 59; P less than .005); and 3) higher incidence of galactorrhea (14 of 50 versus seven of 59: P less than .03). These observations support the contention that paternal deprivation during childhood is associated, in a minority of women, with a predisposition to develop hyperprolactinemia and presumably, prolactinoma later in life. A hypothesis on the possible mechanisms of a casual relationship is presented.