Histochemical studies have shown that endometrial alkaline phosphatase activity in the normal cyclic woman and hormonetreated ovariectomized monkey depends upon stimulation by estr0gen. l It has been demonstrated that a high level of phosphatase activity is present in the surface and glandular epithelium of the estrogen-treated monkey and that subsequent treatment with progesterone results in a diminution of the enzyme. Similarly, the endometrial phosphatase activity is high during the proliferative phase of the human menstrual cycle, diminishes during the secretory phase, and disappears several days before the onset of menstruation. In a recent histopathological study, Gus-berg has presented evidence suggesting a correlation between prolonged estrogen stimulation and the development of endometrial hyperplasia and adenocarcinoma in women. In view of the above findings, the present study was undertaken to determine histochemically the amount and distribution of alkaline phosphatase in hyperplasia and adenocarcinoma of human endometrium.