We prospectively evaluated 75 patients by fiber-optic bronchoscopy and bronchoalveolar lavage (BAL) for the presence of bacterial lower-respiratory-tract infection. BAL specimens were cultured quantitatively for aerobic bacteria, and a cell differential was obtained of the BAL cell population. In 18 "control" patients without evidence of respiratory infection, the presence of greater than 1% squamous epithelial cells (SECs) in the BAL sample accurately predicted the presence of heavy contamination of the sample by oropharyngeal flora. In the remaining "study" patients with potential infection, polymorphonuclear leukocytes were readily identified, and potential lower-respiratory-tract pathogens were recovered in concentrations greater than 10(5) colony-forming units (cfu) per milliliter in 16 of 18 patients with bacterial infection (none had greater than 1% SECs in their BAL sample). No patients without evidence of bacterial infection and with less than or equal to 1% SECs had greater than 10(5) cfu/ml in BAL cultures. These studies establish the ability of BAL techniques to diagnose bacterial respiratory infection.