The present study was designed to assess the relationships between the methods to evaluate adequacy of luteal function. We measured mid-luteal serum progesterone levels by radioimmune assay, luteal phase lengths and mean post-ovulatory basal body temperature rise rates by basal body temperature charts in 40 in-phase-cycle infertile patients and 38 out-of-phase-cycle patients who underwent late-luteal endometrial biopsies at the sterility clinic of Department of Obstetrics and Gynecology, Seoul National University Hospital from Jan. 1986 to Aug. 1986. The results were summarized as follows: 1. No significant differences were found in mean age, mean duration of infertility, mean mid-luteal serum progesterone levels, and mean post-ovulatory temperature rise rate between in-phase-cycle patients and out-of-phase-cycle patients, but significant difference in mean luteal phase length between 2 groups was identified. 2. 91% of total patients sho had luteal phase lengths of less than 11 days showed out-of-phase-cycles. 3. In out-of-phase-cycle group with luteal phase lengths of less than 11 days, 50% had an endometrial lag of at least 4 days, but 10.7% had an endometrial lag of 4 or more days in group with luteal phase lengths of more than 11 days. 4. There was no significant correlation between mid-luteal serum progesterone level and endometrial lag of late luteal phase endometrial biopsy.