A total of 816 women who underwent 1332 cycles of ovarian stimulation for in-vitro fertilization and embryo transfer (IVF/ET) had sonographic assessments of the endometrium within 2 days of oocyte retrieval. Endometrial linings were classified on the basis of thickness and echogenicity, using a grading system described previously. Grades I and IIB ('poor') were associated with a 6% viable pregnancy rate (advanced beyond 12 weeks' gestation) compared with a 29% rate for Grade IIA ('optimal'). In a subset of 112 women with poor endometrial linings during natural cycles, eight out of 21 women (38%) under 40 years of age developed optimal linings following ovarian stimulation with menotrophins, while 19 out of 91 women (21%) aged 41-45 years converted to optimal linings. Twenty-two out of 47 women (44%) who failed to develop optimal endometrial linings following ovarian stimulation converted to Grade IIA during subsequent cycles of exogenous oestrogen replacement. The financial, emotional, and physical burden associated with IVF/ET demands that patients with poor endometrial linings following ovarian stimulation with menotrophins be counselled with regard to either cancelling their cycles of treatment, or having their embryos cryopreserved for transfer to the uterus during a subsequent hormonal replacement cycle.