Quantitative bacteriologic monitoring of bronchial secretions obtained by protected distal catheterism was performed in 60 critically ill patients under mechanical ventilation. A number of factors which have significant bearing on bacterial growth in the respiratory tract were demonstrated. A statistical correlation was found between aspiration pneumonia and presence of Enterobacteriaceae, as well as between hospital-acquired pneumonia and recovery of opportunistic pathogens. S. aureus was especially prevalent in patients under corticosteroids. Aminoglycoside treatments increased the frequency of Acinetobacter infections, whereas prevalence of S. aureus and P. aeruginosa appeared unrelated to the use of any specific antibiotic. The value of bacteriology on protected distal catheter bronchoscopy specimens is to allow accurate diagnosis of lower respiratory tract infections and provide guidelines for the choice of appropriate antibiotics.