In order to clarify the relationship between endometrial histology and progesterone (P<sub>4</sub>), plasma P<sub>4</sub> and estradiol levels in the luteal phase were measured in 126 cases of unexplained infertility. Endometrial biopsies were performed in the midluteal period of menstrual cycles. Forty-three of the 126 cases showed retarded endometrium. Of these 43 cases, 23 exhibited three different types of abnormal P<sub>4</sub> secretion. Type A showed low P<sub>4</sub> levels throughout the luteal period. Type B showed low P<sub>4</sub> levels only in the early luteal period. Type C showed normal P<sub>4</sub> levels in the early luteal period followed by a prompt decline. These findings indicated that P<sub>4</sub> determination during the early, mid- and late luteal phases is necessary to assess P<sub>4 </sub>secretion. However, 20 of the 43 cases had normal P<sub>4</sub> levels through the entire luteal phase, demonstrating an insufficient response of the endometrium to P<sub>4</sub>. Consequently, histological examination of the endometrium is required to investigate the luteal phase defect.