Respiratory tract colonization with Ureaplasma urealyticum in preterm infants has been associated with a higher incidence of pneumonia, severe respiratory failure, bronchopulmonary dysplasia (BPD), and death. In this report, we characterize the lung pathology and expression of tumor necrosis factor-alpha (TNF-alpha) associated with U. urealyticum pneumonia in very low-birth weight infants (VLBW; < or =1500 g). Lung pathology of archived autopsy specimens was retrospectively reviewed in three groups of VLBW infants: 5 gestational controls who died from nonpulmonary causes, 13 infants with pneumonia who were culture and/or PCR negative for U. urealyticum, and 5 infants with pulmonary disease and positive for U. urealyticum by tracheal aspirate and/or lung tissue culture or polymerase chain reaction (PCR). Presence and extent of alveolar macrophages and neutrophils, as well as interstitial lymphocytic infiltration and fibrosis were evaluated. PCR was performed on formalin-fixed, paraffin-embedded lung sections. Additional sections were immunostained for TNF-alpha. The peripheral total white blood cell counts and absolute neutrophil counts were three-fold higher in infants with U. urealyticum pneumonia than cell counts in infants infected with other organisms. There was a trend toward a predominance of neutrophils in alveoli of non- Ureaplasma pneumonia infants, but a trend toward a predominance of alveolar macrophages in U. urealyticum-infected infants. The most striking finding was the presence of increased interstitial fibrosis in all Ureaplasma-infected infants. TNF-alpha immunoreactive cell density was very low in the gestational controls, but increased in both pneumonia groups. We conclude that persistent lung U. urealyticum infection may contribute to chronic inflammation and early fibrosis in the preterm lung.