A case of primary histiocytic lymphoma involving the left temporal lobe and hypothalamus is presented. This lesion was associated with a pituitary microadenoma shown immunohistochemically to contain prolactin and with lactational changes in the breasts. The interruption by the neoplasm of neural pathways from cells producing prolactin inhibitory factor in the hypothalamus and increased production of prolactin by the pituitary are suggested as the probable mechanism. The association of lactational changes in the breasts with primary lymphoma of the brain has not been reported.