OBJECTIVE: To determine hormone levels across the menstrual cycle in women with rigorously defined unexplained infertility. DESIGN: Prospective study. SETTING: National Center for Infertility Research at Michigan. PATIENT(S): Evaluation of 1,885 women with infertility identified 12 women who met the following rigorously defined criteria for unexplained infertility: [1] infertility of > or = 24 months' duration, with no male factor, anatomic or functional disorders of the reproductive tract, or immunologic infertility; [2] normal body mass index (BMI); (3) ovulatory cycles ranging from 26 to 32 days; [4] normal luteal phase determined by endometrial biopsy; and [5] normal baseline hormonal profile. Controls (n = 12) were healthy, parous women with normal ovulatory cycles and normal hormonal screens, and were matched for age and BMI with patients. MAIN OUTCOME MEASURE(S): Daily gonadotropin and steroid hormone levels across the menstrual cycle. RESULT(S): Basal FSH and LH levels in the early, middle and late follicular phases were increased significantly in the group with unexplained fertility compared with the normal controls. The mean (+/-SD) early follicular FSH levels were 7.0 +/- 0.57 mIU/mL in the unexplained-infertility group and 4.7 +/- 0.37 mIU/mL (conversion factor to SI units, 1.00) in the normal controls, respectively. There was no difference between groups over the periovulatory or luteal phase. Midluteal mean (+/-SD) P levels were lower in the unexplained-infertility group than in the normal controls (13.7 +/- 1.6 versus 24.0 +/- 3.2 ng/mL [conversion factor to SI units, 3.180]). Mean E2 concentrations were elevated in the group with unexplained infertility versus normal controls in the early through the late follicular phase but reached significance only in the midfollicular phase. Mean prolactin levels were elevated consistently across the menstrual cycle in the unexplained-infertility group compared with those in normal controls but reached significance only in the early and late follicular and midluteal phases of the cycle. Cortisol concentrations were similar between the two groups. CONCLUSION(S): These data indicate that there are subtle alterations in various hormones measured across the menstrual cycle in women with unexplained infertility compared with those in normal controls, suggesting a diminished ovarian reserve.