Purpose: Ureaplasma urealyticum (UU) colonization in maternal low genitourinary tract can cause preterm labor, spontaneous abortion, and chorioamnionitis. Also preterm babies suffer from perinatal mortality and morbidity. The purpose of this study was to determine the clinical effect of UU colonization in maternal vaginal fluids and premature respiratory secretions on neonatal adverse outcomes. Methods: Retrospective review were performed on preterm baby less than 35 weeks and their mother who were admitted at Kangnam Sacred Heart Hospital, Hallym University from October, 2010 to June, 2012. Maternal vaginal specimen, preterm baby's tracheal fluid and nasopharynx secretion were collected and PCR and/or culture tests were done. The subjects were divided into 4 groups; both maternal and preterm colonization positive were 'UUA' group, only maternal colonization positive as 'UUB', only preterm colonization positive as 'UUC', and both maternal and preterm colonization negative as 'UUD'. Variable clinical outcome were investigated. Results: A total of 170 data were collected from the preterm delivery mother and their babies. The overall prevalence of maternal UU was 51% (86/170) and the prevalence of neonatal colonization was 39% (66/170). Of those 4 groups, gestational age (28.5±3.0 week, P =0.000) and birth weight (1,208.8±451 g, P =0.000)were the smallest in UUA. Also, prevalence of histological chorioamnionitis (P =0.040) and bronchopulmonary dysplasia (P=0.000) were statistical higher in UUA. Conclusion: Maternal colonization of UU was associated with histological chorioamnionitis. Maternal UU colonization and preterm respiratory secretion was associated with bronchopulmonary dysplasia. [ABSTRACT FROM AUTHOR]