Plasma progesterone concentrations drawn at the time of endometrial biopsy in 26 infertility patients with histologically documented luteal phase inadequacy were compared with those of 26 infertility patients with normal biopsies. Although as a group the former patients had lower progesterone values and shorter cycles, there was considerable overlap. Therefore, although plasma progesterone determinations and temperature charts are useful in the detection of ovulation and in the interpretation of the biopsy results, a properly obtained endometrial biopsy is essential for the diagnosis of luteal phase inadequacy.