Tests for the possible presence of Ureaplasma urealyticum (U. u.), a type of mycoplasma transmitted perinatally, were performed on 53 consecutive mature newborns and a group of 108 predominantly premature infants. Standard microbiological tests were performed on smears from throat, nose and ear, as well as from vagina or anus. U. u. was isolated from 15 of the 53 mature newborns (28%): none of the children was ill. In the premature group the microorganism was isolated from 13 of the 108 infants (12%). Isolation was the more frequent the smaller the birth weight and the shorter the gestational period. The carrier rate was 50% (6 of 12) in those with birth weights under 1,500 g and a gestational period of less than 29 weeks. In all 6 newborns under 1,500 g who carried U. u. bronchopulmonary dysplasia developed, but in only 2 of the 9 in whom U. u. was not isolated. Administration of erythromycin (20-45 mg/kg . d intravenously) improved the clinical course of 3 of the 5 affected with bronchopulmonary dysplasia. - The results demonstrate that U. u. is of significance in perinatal medicine and should be considered especially in the treatment of premature infants requiring artificial ventilation.