AT least 15 per cent of all married couples are involuntarily infertile.1 During the past several decades, infertility due to peritoneal and endocrine factors has received the greatest attention, and specific treatments for many of these causes are now available. Previously, little credence was given to the possibility that low-grade, asymptomatic infection affects human fertility, but there is now a growing belief that such infections can influence the incidence of conception, implantation, and embryonal death. Considerable disagreement exists concerning the role of Ureaplasma urealyticum in human infertility.2,3 The inconclusive results of previous studies may be explained in part by failure. © 1983, Massachusetts Medical Society. All rights reserved.