In order to examine the accuracy of endometrial dating, endometrial dating from 52 infertile women with normal luteal function was investigated. Dating from the histological findings was evaluated from three time points, the onset of the next menstrual period (NMP), the last day of the low phase of basal body temperature (BBT) and the urinary LH peak determined with the "Organon LH Color" kit (LH). Beside microscopic observation, morphometric analysis of endometrial findings was done by means of a high speed color image analysis system "Olympus SP-500". On the day of LH Color (+2), the serum LH level showed a maximum of 35.5 +/- 12.6 mIU/ml (Mean +/- S.D.) (n = 27). The correlation coefficients for histological dating and chronological dating based on NMP, BBT and LH were 0.685, 0.728 and 0.879 in gland and 0.620, 0.737 and 0.833 in stroma, respectively. The multiple correlation coefficient for histological dating evaluated by morphometric analysis and chronological dating based on LH was 0.959. So we conclude that endometrial dating evaluated by morphometric analysis seems to be the most objective and useful.