A number of reports indicate that many women experience minor physical and psychological symptoms just prior to or during menses. Rarer episodic psychotic states associated with menses have received scanty attention. In the few available case reports, many hypotheses about their etiology have been advanced, usually unsupported by systematic studies.A 15-year-old girl with periodic psychotic episodes associated with menses was studied over many months. The interval between her periods was around 45 days. Psychotic episodes usually started a few days before or in association with menstural bleeding. Typically characterized by withdrawal, negativism, an apprehensive attitude, and suspiciousness, they remitted fully in 10 to 12 days with or without treatment. To facilitate necessary behavioral observations and complete various investigations, the patient was kept for nearly 60 days without treatment as an inpatient. Repeated vaginal smear cytology examinations during the menstrual cycle indicated a predominant estrogen influence throughout the cycle with an absence of the expected progesterone predominance in its later part. The patient gained 5 pounds between two cycles. Therapy with progesterone was instituted following the observation period. This patient's Rorschach revealed strong conflicts in relation to her sexuality. Concomitant with change in vaginal cytology reflecting a more balanced estrogen/progesterone influence, the patient stopped having psychotic episodes with menses. During a trial period when progesterone therapy was stopped, the psychotic episodes recurred but were prevented with its subsequent reinstitution.A complex interplay of hormonal factors that brings heightened sexual drive and possibly water retention in a predisposed patient can contribute to the causation of psychotic episodes at menstrual onset. Menstrual bleeding can assoociatively facilitate sexual fantasies and heighten conflict. Under this combined stress, the ego's defensive structure is likely to crumble with resultant psychotic episodes.