Serial tracheal cultures were made in 129 patients who required tracheal intubation for 48 hours or longer. Twenty-eight patients were managed with endotracheal tubes and 101 had tracheostomy. Mechanical ventilation was required by 113 patients. Tracheal cultures became positive for pathogenic bacteria or fungi in 115 patients, and species of Pseudomonas, Klebsiella, Candida, and Proteus predominated. Replacement of one pathogen by another occurred frequently, and 43% of the patients became colonized with two or more gram-negative organisms. Tracheobronchitis occurred in 31 patients; 47 had pneumonitis; and 28 developed colonization without evidence of respiratory infection. Antibiotic overuse often resulted in infection with resistant organisms. The data suggest greater effort in distinguishing colonization from respiratory infection, with preferential omission of systemic antibiotics for simple colonization or tracheobronchitis.