This study was designed to investigate the role of three-dimensional (3D) endometrial ultrasound in predicting the outcome of an in-vitro fertilization (IVF) programme. In 47 IVF cycles measurements of endometrial thickness and volume, as assessed by 3D transvaginal ultrasound on the day of oocyte retrieval, and concentrations of oestradiol and progesterone in the same patient sampled on the day of sonography, were related to the occurrence of a successful implantation. The overall pregnancy rate was 31.9% (15/47). Fifteen pregnant patients had a mean endometrial thickness and volume of 10. 8 +/- 2.3 mm (mean +/- SD) and 4.9 +/- 2.2 ml, respectively. Thirty-two non-pregnant patients had corresponding measurements of 11.8 +/- 3.4 mm and 5.8 +/- 3.4 ml respectively. Endometrial thickness varied widely in both groups, in pregnant patients from 6. 9 to 16.0 mm, in non-pregnant patients from 6.5 to 21.1 mm. Oestradiol concentrations were not significantly correlated with either endometrial thickness or volume. The conclusion from the present data is that 3D volume estimation of the endometrium as well as analysis of endometrial thickness on the day of oocyte retrieval had no predictive value for conception in IVF cycles.