Background: Ureaplasma urealyticum (Uu) colonization in preterm infants has been associated with the development of chronic lung disease. We investigated Uu colonization in infants ≤ 30 weeks GA and the effect of treatment with erythromycin. Subjects: 155 ventilated preterm infants (mean GA 26.2 ± 1.5 weeks) were cultured for Uu in endotracheal aspirate. Interventions: Uu+infants (n=29) were randomly assigned to no treatment or erythromycin 40 mg/kg/day. i.v. or p.o .Results: rate of colonization was 29/155 (20%). Uu+infants had lower GA than the Uu- infants (25.4 vs 26.4 weeks: p< 0.005). For Uu+infants PROM(p < 0.001), chorioamnionitis (p<0.001) and vaginal delivery(p<0.0001) were more common. More Uu+infants needed extra O2 at 36 weeks (p<0.05). Treatment reduced colonization in 12/15 (80%) Uu+infants. Time with supplemental O2 or number of infants with O2 at 36 weeks were not significantly reduced in treated infants. Extra O2 requirement at 36 weeks was related to lower GA, appearance of PDA and number of sepsis episodes. Conclusion: Uu colonization is related to lower GA. PROM, chorioamnionitis and vaginal delivery. Treatment with erythromycin reduced colonization but did not significantly affect time with supplemental O2 or a need for O2 at 36 weeks postconceptual age.