Premenstrual syndrome (PMS) is a menstrual cycle-related disorder of mood and behavior with unknown etiology and lack of evidence of endocrine imbalances. The absence of menstrual cyclicity eliminates PMS symptoms, and cyclic gonadal changes appear necessary but not sufficient cause for behavioral symptoms in predisposed women. As there are many depressive symptoms in PMS, we focused on the neuroendocrine characteristics of depression, primarily hypothalamic-pituitary-adrenal hyperactivity, and investigated plasma adrenocorticotropin (ACTH) and cortisol levels in the menstrual cycle. Blood samples were drawn daily from ten patients with PMS and eight asymptomatic control subjects who also reported symptoms daily throughout one menstrual cycle. The ACTH levels were significantly lower in patients with PMS compared to control subjects in the luteal phase. Two PMS subgroups were identified by cluster analysis applied to symptom scores. Plasma ACTH levels of PMS Cluster 1 subjects were significantly lower compared to controls. The ACTH levels of PMS Cluster 2 subjects were nearly identical to controls. These preliminary results suggest distinct hormonal subgroups in the heterogeneous PMS population and support further study of ACTH as a biological correlate of PMS.