An endometrial biopsy and a blood sample for progesterone determination obtained simultaneously in the midluteal phase of the cycles of 55 infertile women were compared for reliability for confirmation of presumptive ovulation and evaluation of luteal function. Progesterone levels of 3 ng/ml or greater were found in 90.5% of the cycles. Secretory endometrium was identified in 81% of the cycles. Thirty-three cycles yielded sufficient information to compare the two methods for evaluation of luteal function. Histology and progesterone levels were consistent with each other and the presumed time of ovulation in only 11 cycles. Histology was inconsistent with the presumed time of ovulation in 20 cycles, while progesterone was inconsistent in only two cycles. Additional samples for progesterone determinations were obtained during the biopsy cycles of 15 patients who presented adequate data for evaluation of luteal function. A single, well-timed progesterone determination appeared adequately to reflect the data obtained from serial samples in the same cycle. These results support the thesis that a single, well-timed serum progesterone determination is superior to a single endometrial biopsy as a screening method for confirmation of presumptive ovulation and for evaluation of luteal function.