OBJECTIVE: To determine the ability of luteal phase length determined by basal body temperature (BBT) pattern and a midluteal serum progesterone level to predict the result of an endometrial biopsy in a subsequent cycle. METHODS: We performed a retrospective analysis of 141 women with a history of infertility who were being evaluated for luteal function. The luteal phase length determined from a BBT chart of one menstrual cycle was compared to a single midluteal serum progesterone level from a second menstrual cycle. These findings were compared to a luteal phase endometrial biopsy performed in a third menstrual cycle. Subjects were divided into four groups depending upon luteal phase length (normal 11 or more days) and serum progesterone level (normal at least 10 ng/mL). The four groups were designated "normal," "short luteal phase," "low progesterone," and "abnormal," depending upon the results of the two tests. The frequency of in- and out-of-phase endometrial biopsy results in the four groups was compared. RESULTS: There was no difference in the occurrence of an in- or out-of-phase endometrial biopsy when the four groups were compared. CONCLUSION: Neither luteal phase length nor a single midluteal serum progesterone level was predictive of subsequent in-phase or out-of-phase endometrial biopsy.