OBJECTIVE To determine the effect of the presence of Ureaplasma urealyticum in semen on IVF outcome (fertilization, pregnancy, and abortion rates). METHODS Prospective study. METHODS Private IVF unit in Athens, Greece. METHODS One hundred ninety-one asymptomatic men with normal semen parameters whose wives underwent an IVF cycle. METHODS Culture of semen for U. urealYticum on the day of oocyte retrieval. METHODS Fertilization, pregnancy, and abortion rates after IVF. RESULTS Ninety-six (86%) of the 112 women whose husbands' semen was negative for U. urealyticum and 65 (82%) of the 79 women whose husbands' semen was positive for U. urealyticum underwent ET. The pregnancy rate (PR) was 20% (19/96) in the negative group and 17% (11/65) in the positive group. An increased incidence of abortions (6/11) was observed in the positive group (abortion rate, 54%), compared with 21% (4/19) in the group of women whose husbands' semen was negative for U. urealyticum. CONCLUSIONS Fertilization rates and PRs may not be affected by the presence of U. urealyticum in semen on the day of oocyte retrieval. It can be presumed that the semen preparation for IVF cleanses the semen of U. urealyticum. On the other hand, the higher abortion rate in the U. urealyticum-positive group might be related to maternal factors, such as an existing U. urealyticum infection or one contracted after conception.