Basal body temperature (BBT) charts from three menstrual cycles of 20 normal women and 20 women with biopsy-proven luteal phase defect (LPD) were reviewed. Mean luteal phase length in the normal women was 13.4 days, and that of the women with LPD was 11.8 days (P less than 0.05). Six (30%) of the LPD patients had luteal phases of less than 11 days according to BBT, and five of these patients had severely out-of-phase endometrial biopsies. None of the normal patients had luteal phases of less than 11 days. There was no significant difference in the mean rate of postovulatory BBT rise between the two groups. It is suggested that the slope of postovulatory temperature shift is not helpful in the diagnosis of LPD but that evidence of a luteal phase of less than 11 days on BBT does indicate a high likelihood of LPD.