We assessed the frequency and the potential role of respiratory viruses on disease outcomes in hospitalized patients and lung transplant recipients who underwent a bronchoalveolar lavage (BAL) for an acute respiratory infection. BAL specimens (148) were analyzed by reverse transcription-polymerase chain reaction for the presence of 11 different viruses, as well as Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella pneumophila. Respiratory viruses were identified in 34 of 117 BAL specimens (29%) obtained in patients with a suspected respiratory infection and in only 2 of 31 control subjects (7%) (p < 0.01). M. pneumoniae was identified in five additional cases. Only 30% of cases that were virus positive by molecular methods were also positive by cell culture analysis. Rhinovirus was the most frequently identified virus (56% of cases) followed by respiratory syncytial virus (27%). In lung transplant recipients, the rate of viral infections was 55% in cases with respiratory symptoms compared with only 4% in control subjects (p < 0.001). In these cases, respiratory viral infections were associated with significant lung function abnormalities. By using reverse transcription-polymerase chain reaction assays, we frequently identified respiratory viruses in BAL specimens of patients hospitalized with lower respiratory tract infections. These viruses were associated with high morbidity, particularly in lung transplant recipients.