We evaluated the effect of antibiotic treatment received before the suspicion of pneumonia on the diagnostic yield of protected specimen brush (PSB), direct examination (BAL D) and culture (BAL C) of lavage fluid on consecutive mechanically ventilated patients with suspected nosocomial pneumonia. Bronchoscopy was always performed before any treatment for suspected pneumonia. One hundred and sixty-one patients with suspected pneumonia underwent PSB and BAL before any institution or change in antibiotic therapy (AB). Sixty-five patients received AB for an earlier septic episode (ON AB group) and 96 patients did not (OFF AB group). All but two strains recovered were highly resistant to previous AB. Sensitivity and specificity of each test were not different between the ON AB and OFF AB groups as well as the percentage of complete agreement between the 3 procedures, 74 and 67% respectively. We conclude that previous AB received to treat an earlier septic episode unrelated to suspected pneumonia do not affect the diagnostic yield of PSB and BAL.