Objective:To study the clinical characteristics of ureaplasma urealyticum (UU) infection in preterm infants with gestational age <34 weeks.Methods:From January 2017 to December 2021, premature infants with gestational age <34 weeks admitted to neonatal department of our hospital were enrolled in this prospective cohort study. UU-DNA from respiratory tract samples were examined using quantitative fluorescence polymerase chain reaction method. The infants were assigned into UU (+) group and UU (-) group. Perinatal factors and clinical characteristics were compared between the two groups.Results:A total of 182 preterm infants were enrolled, including 59 cases (32.4%) in UU (+) group and 123 (67.6%) in UU (-) group. UU (+) group had significantly lower gestational age and birth weight and significantly higher incidences of vaginal delivery, premature rupture of membranes (PROM) >18 h and maternal chorioamnionitis than UU (-) group ( P<0.05). Compared with UU (-) group, UU (+) group had significantly higher leucocyte count, neutrophil count and interleukin-6 at 1, 24 and 72 h after birth ( P<0.05). No significant differences existed in C-reactive protein and procalcitonin between the two groups at each time point ( P>0.05). In UU (+) group, the incidences of intrauterine pulmonary infection and bronchopulmonary dysplasia (BPD) were higher and the incidence of respiratory distress syndrome was lower than UU (-) group ( P<0.05). No significant differences existed in the incidences of intraventricular hemorrhage, periventricular leukomalacia, feeding intolerance, necrotizing enterocolitis, retinopathy of prematurity between the two groups ( P>0.0 5). UU (+) group had significantly longer duration of oxygen therapy than UU (-) group ( P<0.05). No significant differences existed in the duration of invasive mechanical ventilation and hospital stay between the two groups ( P>0.05). Conclusions:Preterm infants <34 weeks with positive UU in respiratory tract secretions have higher incidences of vaginal delivery, PROM>18 h and maternal chorioamnionitis. Leukocyte and neutrophil count and interleukin -6 are higher in these infants. They need prolonged oxygen therapy and have increased risks of intrauterine pulmonary infection and BPD.