To evaluate immunoreactive (IR) inhibin as a marker of corpus luteal function, 134 women with natural ovulatory cycles were studied. Serum IR-inhibin levels during the luteal phase were significantly lower in women with low progesterone (P4) levels (less than 10 ng/ml). Endometrial dating correlated more with serum P4 than IR-inhibin levels. All 14 conceiving cycles showed serum P4 levels greater than or equal to 10 ng/ml and IR-inhibin levels greater than or equal to 3.67 IU/ml during the midluteal phase. Furthermore, IR-inhibin levels in the early follicular phase seemed to be associated with lowered P4 levels in the ensuing luteal phase. These results suggest that IR inhibin may be closely related to corpus luteum function.