The endometrial response to a standard hormone replacement therapy in 18 women with premature ovarian failure was examined by serial ultrasonographic measurement of endometrial thickness and histological study of endometrial biopsy taken on day 19 of the cycle. Women with idiopathic ovarian failure (n = 10) had significantly better response than women with Turner's syndrome (n = 4), whereas women with premature ovarian failure associated with previous chemotherapy (n = 4) had an intermediate response. These observations suggest that the endometria of women with Turner's syndrome responded suboptimally to steroid hormones. However, all endometrial biopsies studied revealed secretory changes. Overall, the results of histological dating of endometrial biopsy were found to be positively correlated with endometrial thickness on day 19 of the cycle (r = 0.72, P less than 0.01). The plasma concentration of oestradiol on days 15, 19 and 29 of the artificial cycle were found to be significantly higher than those on the corresponding days of the natural cycle.