The incidence and outcome of Ureaplasma urealyticum infection were studied in 98 infants born before 34 weeks of gestational age. Infection was defined as the presence of one or more isolations of U. urealyticum in samples obtained from trachea, blood, cerebrospinal fluid, or postmortem brain or lung biopsies. Forty-seven infants were infected. Intact amniotic membranes had no protective effect against infection; intrauterine U. urealyticum infection was detected in 19 infants who were born by cesarean section with intact amniotic membranes. Respiratory distress syndrome, the need for assisted ventilation, severe respiratory insufficiency, and death were significantly more common among infected than among noninfected infants. Our results suggest that U. urealyticum infection is associated with an unfavorable short-term outcome in preterm infants.