We evaluated the predictive value for pregnancy of the endometrial thickness and pattern assessed by vaginal sonography on the day of human chorionic gonadotrophin (HCG) injection and the day of embryo transfer in 74 stimulated cycles for in-vitro fertilization (IVF) which were analysed prospectively. Thickness was measured from the echogenic interface of the endometrial-myometrium junction in transverse fundal sections. The distribution of endometrial pattern on the day of HCG was 19 A (poor quality) cases (25.7%) and 55 B (good quality) cases (74.3%). On the day of embryo transfer, 16 out of the 19 A cases (84%) remained as A pattern, while the remaining three cases (16%) had changed to B pattern; of the 55 B cases, 29 (53%) remained the same, while 26 cases (47%) changed to A pattern. There was no significant correlation between the endometrial pattern on the day of HCG and/or on the day of embryo transfer and the peak serum oestradiol levels, the number of preovulatory oocytes aspirated, the serum progesterone levels and oestradiol: progesterone ratio on the day of transfer. In contrast, on the day of HCG, endometrial thickness correlated with serum oestradiol levels on that day (P = 0.02). The presence of pattern A on the day of HCG was associated with a lower pregnancy rate [three out of 19 cases, (15.8%)], compared to pattern B [16 out of 55 cases, (29.1%)], although this was not statistically significant (P greater than 0.05). On the day of embryo transfer, comparable pregnancy rates between A pattern [11 out of 42 cases, (26.1%)] and B pattern [eight out of 32 cases, (25%)] were found.(ABSTRACT TRUNCATED AT 250 WORDS)