The sonographic appearance of the endometrium was evaluated in 97 infertile women, 51 from a fertility unit (FU) and 46 from an in vitro fertilization (IVF) program. Three groups of patients were identified: those with normal findings, those with an incomplete endometrial echo, and those without any endometrial echo. Findings were correlated with the type of ovulation induction protocol, the peak estradiol level, and the number of successful pregnancies. In the IVF group, 72% of the patients did not exhibit normal endometrial echoes but had adequate estradiol levels, compared with 62% of the FU patients with normal endometrial echoes but significantly lower estradiol. The data suggest that IVF patients have a nonresponsive endometrium rather than inadequate estrogen stimulation and therefore that an endometrial abnormality may be an important cause of their infertility. Ultrasound may be used as a screening method to identify such patients.