In this study, the prevalence of retarded endometrial development in the luteal phase of an infertile population (20/142 = 14%) was found to be significantly higher (P less than 0.05) than in that of a control, fertile population (3/68 = 4.4%). However, when the infertile subjects were subdivided into groups of different aetiology, it was found that women with endometriosis and unexplained infertility had a significantly higher prevalence of retarded endometrium (6/21 = 29%, 10/48 = 21% respectively; P less than 0.01 in both cases) than women with normal fertility; whereas the prevalence in women with tubal or male infertility (1/34 = 2.9% and 3/39 = 7.7%, respectively) was not significantly different from that in the fertile subjects. There were no significant differences in the progesterone profiles of the four groups of infertile subjects and the group of fertile subjects. Of the 15 cases of retarded endometrial development with known progesterone profile, two were associated with subnormal progesterone whereas the remaining 13 were associated with normal progesterone, the latter suggesting an abnormal response of the endometrium to a normal amount of progesterone. The findings suggest that further morphological study of the endometrium should be aimed at subjects whose infertility is unexplained or associated with endometriosis, in whom the prevalence of abnormal endometrium is increased.