In order to assess the bacteriology of bronchial secretion samples, 40 patients in the intensive care unit who had developed fever and pulmonary infiltrates during mechanical ventilation have been studied. In each patient bronchial secretion samples were obtained by a double lumen distally-plugged telescopic catheter (DTC) inserted under direct view through the fiber bronchoscope (FB) as well as from simple bronchial aspiration (SBA) done simultaneously. A week later DTC and SBA were repeated. A statistically significant difference was found between the positive cultures obtained by SBA and those obtained by DTC (p less than 0.005). However, in 9 samples (14.27%) other microorganisms were isolated with DTC which were not detected by SBA and a lower number of colonizing microorganisms were found by DTC (p less than 0.05). The isolation of microorganisms by DTC allowed more precise management and moreover, a better clinical course was observed in those patients in whom chemotherapy was based on the data given by DTC. The relationship between the cultures obtained by DTC and the previous antibiotic treatment was statistically significant, finding a greater number of positive cultures when they were taken 2 hours after the last doses of the antibiotic. This relationship was not found in the cultures obtained with SBA. The most frequently isolated microorganisms were diverse types of Pseudomonas and Acinetobacter. No complication caused by the techniques arose.