Objective: To identify demographic, clinical, and laboratory variables predictive for a concurrent bacterial pulmonary infection in ventilated infants with respiratory syncytial virus (RSV) lower respiratory tract disease (LRTD) and investigate antimicrobial drug use. Design and setting: Retrospective, observational study in a 14-bed pediatric intensive care unit. Patients: 82 infants younger than 1 year of age with a virologically confirmed RSV LRTD during 1996-2001, of whom 65 were mechanically ventilated. Results: Microbiological data were available from 38 ventilated infants, 10 of whom had a positive blood culture (n=1) or endotracheal aspirate (n=9) obtained upon admission to the pediatric intensive care unit (PICU). Infants with a positive culture had a lower mean gestational age but were otherwise demographically comparable to those with negative culture results. Infants with a positive culture were ventilated 4 days longer. Indicators for a concurrent bacterial infection were comparable between ventilated and nonventilated infants. Antimicrobial drugs were used in 95.1% of infants (100% of ventilated infants) with a mean duration of 7.8&PLUSMN; 0.3 days. The moment of initiation and duration of antimicrobial drug treatment varied considerably. Conclusions: We observed in ventilated infants a low occurrence of concurrent bacterial pulmonary infection, but infants with positive cultures needed prolonged ventilatory support. Improvement in the diagnosis of a pulmonary bacterial infection is warranted to reduce the overuse of antimicrobial drugs among ventilated infants with RSV LRTD and to restrict these drugs to the proper patients.