Observations were made on 817 menstrual bleedings of 221 patients suffering from schizophrenic and affective disorders. A more detailed study has been made in 31 cases by an analysis of the vaginal smears. A greater irregularity in menstrual interval than in a comparable normal group was found. A tendency to a delay, a weakened expression, or a temporary suppression of the follicular reaction was noted. Prolongation of the menstrual interval or amenorrhea was frequent. Short cycles also were observed. All these abnormalities are interpreted as the result of an adverse effect upon the growth of the ovarian follicles. A correlation between the severity of the illness and the degree of abnormality of the menstrual cycle was found. An improvement in the mental condition was usually accompanied by a change to a more normal menstrual function. The existence of an etiological relationship could not be ascertained.