To evaluate the role of endometrial thickness and pattern in in-vitro fertilization (IVF), these parameters were prospectively measured in 516 cycles of IVF with embryo transfer at our clinic. Pregnancy and embryo implantation rates were assessed for each mm of endometrial thickness and for each of three endometrial patterns. Embryo implantation, clinical and ongoing pregnancy rates were significantly higher in the patients with an endometrial thickness > 9 mm (24.4, 48.6 and 42.2% respectively) compared with those of < 9 mm (14.3, 16.0 and 11.7% respectively; P < 0.005). Endometrial thickness was negatively influenced by age and positively influenced by oestradiol concentration. The majority of patients (69.8%) exhibited a 'ring' endometrial pattern. Embryo implantation and clinical pregnancy (statistically significant), as well as ongoing pregnancy rates (not statistically significant), were lower in patients exhibiting the 'solid' pattern. Endometrial thickness is independent of pattern in its effect on pregnancy outcome. In conclusion, endometrial thickness > 9 mm as well as ring and intermediate endometrial patterns denoted a more favourable prognosis for pregnancy in IVF but thinner endometrium and those exhibiting a solid configuration had an acceptable pregnancy outcome.