A comprehensive study on 9 patients with a history of amenorrhea persisting for 5 months or more is presented. Endometrial biopsies taken at weekly intervals made it possible to divide these patients into 2 classes as follows: 1.1. Amenorrhea With Persistent Proliferative Phase Endometrium.—Patients with this type of amenorrhea are usually true endocrine problems, their pathologic physiology being a pituitary-ovarian failure with absence of ovulation. The proliferative endometriums vary from the atrophic proliferative type to that of cystic hyperplasia. The endometrial pattern for each individual patient remains amazingly constant. This suggests that the estrogen level in these individuals is a fixed one and in the absence of any cyclic variation of this hormone level, the spiral end-arteries of the endometrium are denied their normal stimulus for bleeding. It is the estrogen-progesterone withdrawal which provides the end-artery stimulus in the normal woman. Satisfactory treatment for this type of amenorrhea is available in the form of stilbestrol 5.0 mg. daily for 10 days supplemented during the last 5 days with progesterone 5.0 mg. Uterine bleeding follows in 3 to 4 days in most cases and the course of treatment is repeated on the fifteenth to twenty-fifth day of this artificially induced cycle. Immediate results were good in that 6 of 7 patients menstruated immediately following treatment and 5 of them continued to menstruate without treatment for variable periods of time. An interesting observation is that despite the induction of bleeding, there occurred little change in the endometrial pattern following treatment. Comparison of the photomicrographs in Fig. 2 shows little difference before treatment and after menstruation had been induced.2.2. Amenorrhea With Persistent Secretory Phase Endometrium.—A type of amenorrhea rarely diagnosed is that caused by a persistent corpus luteum. Two patients are reported with long periods of amenorrhea and on whom persistent secretory endometriums were obtained. Both patients were originally diagnosed as pregnant and, indeed, one was found to have a positive Friedman test. Both resumed normal menstrual cycles after excision of a small corpus luteum in the right ovary of one, and the removal of a corpus luteum cyst in the left ovary of the other after 14 and 6 months' amenorrhea respectively. Undoubtedly, there are some cases in which a persistent corpus luteum undergoes atrophy spontaneously with re-establishment of the normal cycle. However, those patients with persistent amenorrhea in whom repeated secretory phase endometriums are obtained, a diagnosis of persistent corpus luteum or corpus luteum cyst can be made and the treatment is excision of the corpus luteum.