OBJECTIVE The objective of the study was to characterize the changes that occur in chest radiographs at the time of gram-negative bacilli (GNB) nosocomial bloodstream infection (BSI) and to determine the contribution of bronchopulmonary dysplasia (BPD) and airway gram-negative bacterial pathogens to the clinical diagnosis of GNB nosocomial pneumonia. METHODS This retrospective investigation involved 41 BSI infants (study group) and 50 GNB airway colonized infants who had sepsis workup with negative blood cultures (control group). We correlated clinical findings (95 blood and 305 tracheal aspirate (TA) cultures) with radiographic findings noted within 2 days before, the day of, and the day after blood cultures. Two radiologists independently graded 258 films using a modified score for BPD and a semiquantitative score ("probable," "possible," or "unlikely") for pneumonia. RESULTS Mean birth weight was 1057 gm and 1044 gm for the study and control groups, respectively. Of the study population, 54% were male, 21% were black, 89% received surfactant, 79% received dexamethasone, and 88% survived. Average age at the time of blood cultures for both groups was 23 days. Most common isolates from blood and TA were Klebsiella pneumoniae, Enterobacter cloacae, Escherichia coli, and Pseudomonas aeruginosa. Eight BSI infants died, mainly as a result of P. aeruginosa and K. pneumoniae; three control patients also died. Radiological findings were that BPD scores did not change in relation to BSI and were similar between study and control groups. Of the BSI patients, 21 presented with newly positive TA at the time of positive blood culture; "probable" or "possible" pneumonia was diagnosed in 18 of them. Their BPD scores were higher than those of the remaining BSI patients, of whom seven were already airway colonized, nine had negative TAs, and four were not intubated. Only one of these 20 patients had "possible" pneumonia noted on chest x-ray films. CONCLUSIONS Radiographic signs of air space disease accompanied by the recovery of GNB respiratory pathogens from the blood and from a previously uncolonized airway strongly support the clinical diagnosis of GNB nosocomial pneumonia. Radiological signs of BPD are stable in relation to nosocomial BSI caused by GNB, but BPD radiological scores are higher among infants who also had a newly acquired respiratory GNB. BSI, new respiratory tract GNB, and BPD are critical associations for the clinical interpretation of radiographic changes in the ventilated newborn.