Objective: To determine whether serum inhibin A and inhibin B concentrations are lower in patients with luteal dysfunction than in women with normal luteal function. Methods: Serum samples were collected from seven healthy women with regular menstrual cycles. Serum samples on days +5 to +9 after the LH surge were collected from patients with luteal dysfunction. The diagnosis of luteal dysfunction was based on a luteal phase duration less than 11 days and a single midluteal progesterone level below 10 ng/mL. Serum levels of inhibin A, inhibin B, progesterone, estradiol (E2), FSH, and LH were measured. Results: The serum inhibin A levels were increased toward the late follicular phase. The levels reached a maximum during the midluteal phase, followed by a fall during the late luteal phase. The serum inhibin B levels were high during the follicular phases and the early luteal phase. The levels decreased during the midluteal and late luteal phases. Serum levels (mean t standard error of the mean) of inhibin A in patients with luteal dysfunction were significantly lower than those in women during the midluteal phase (26.2 α 2.9 compared to 41.9 α 2.8 pg/mL; P < .01) in addition to the expected decrease in serum progesterone levels (6.3 ± 0.7 compared to 14.7 ± 1.2 ng/mL; P < .01). Serum inhibin B levels did not differ significantly between normal women and those with luteal dysfunction. There also were no significant differences in the E2, FSH, and LH levels. Conclusion: Levels of inhibin A, but not of inhibin B, may reflect the human luteal function.