This study analysed the relation between daily salivary progesterone profiles in the luteal phase and endometrial development as assessed by morphometric studies of endometrial biopsies that were timed by luteinizing hormone surge. Among 34 fertile women studied, a threshold relation was noted between cumulative salivary progesterone concentration and 11 of the 17 morphometric features measured, as well as the results of the histological dating. Above the observed threshold, a significant linear correlation was found between cumulative salivary progesterone and the results of histological dating. Among 55 women, progesterone profile in the first half of the luteal phase, but not the second half, was found to be significantly lower in those with retarded endometrial development than in those with normal development. Overall, 50% of retarded endometrial development was associated with a subnormal progesterone profile, but the other 50% was associated with a normal progesterone profile, in keeping with an abnormal response of the endometrium due to progesterone receptor deficiency. Retarded endometrial development was more likely to be associated with a subnormal progesterone profile in the first half of the luteal phase than in the second half.