Dating of maturity of the endometrium by histologic examination was correlated with four methods of ovulation detection in 13 cycling parous women. Histologic dating was assessed independently by two pathologists and correlated with the postovulatory duration as determined by daily transvaginal ultrasound scanning, serum LH measurements, basal body temperature (BBT), and subtraction of 14 days from the onset of menses. In addition, progesterone and estradiol (E2) were measured in daily serum samples. Dating of the endometrial biopsy was highly correlated (P<.002) with the day of ovulation as determined by ultrasound, and was found to be within 2 days of the correct postovulatory day on evaluation of 25 of 26 (96.1%) of the interpretations. The accuracy of dating using the LH surge was 84.6% (22 of 26 interpretations), and with the BBT thermogenic shift was 76.9% (20 of 26 interpretations). However, dating of the endometrium was within 2 days of the correct day in only 17 of the 26 interpretations as determined by subtracting 14 days from the onset of the subsequent menses. The accuracy of dating was significantly better correlated (P<.025) with days from ovulation as determined by ultrasound than as calculated from the onset of menses. There was a significant correlation between endometrial dating and the amount of progesterone (P<.01) and E2 (P<.01) secreted from the day of ovulation, as determined by transvaginal ultrasound, to the day of biopsy. These data confirm a strong correlation between endometrial dating and ovarian hormone secretion during the postovulatory phase. If histologic dating of the endometrium is used to diagnose luteal insufficiency, it must be correlated with days since ovulation as determined by an accurate method rather than as calculated by subtracting 14 days from the onset of menses