Twenty-one patients on mechanical ventilators for greater than 48 hours who had new localized infiltrates were evaluated using a quantitative culture technique of the involved lung compared to the non-involved lung. Based on the clinical course, response to antibiotics, or subsequent analysis of pathologic specimens, eight patients were felt to have acute bacterial pneumonia, while the remaining 13 were felt to have an alternative cause of their infiltrate. Cultures of the protected brush specimen of the involved lung in all eight cases of bacterial pneumonia had one or more organisms grown at a greater than 100 colony forming units (cfu) per ml while only one of the 13 cases of non-pneumonia had a culture from the involved area having greater than 100 cfu per ml (p less than 0.001). The non-involved area always grew fewer organisms than the involved area, and in 16 cases, there was no growth from the specimen obtained from the non-involved area.