Ureaplasma urealyticum (Uu) infection in pregnancy can lead to a poor outcome. To confirm the relation between Uu and intrauterine infection, 65 placentas were taken for isolation and identification of Uu from pregnant women with positive antibody to Uu. Uu was isolated from 13 (20%) of the 65 placentas. The pathologic examination of 6 placentas showed that there were acute or chronic inflammatory cell infiltration. Cord blood IgM antibody was examined by ELISA from 5 cases in which Uu was isolated from placentas and 3 of them was positive to Uu. The 5 cord blood specimens were assayed by the metabolic inhibition test for serotypes of Uu, and one of them was serotype 8. Since the isolation of Uu from some pregnant women, the pathologic changes of placentas, and the detection of specific IgM from cord blood showed a good consistency, we conclude that the Uu infection may occur in utero.