Serial tracheal cultures for aerobic and anaerobic bacteria were obtained from 27 pediatric patients during one year of follow-up. The patients had required tracheostomy and prolonged intubation for periods ranging from 3 to 12 months (average, 7 1/2 months). Cultures of tracheal aspirates yielded 1,508 isolates of pathogenic aerobic (969 isolates) and anaerobic (539 isolates) bacteria. The most frequent aerobic isolates were Streptococcus pneumoniae and Staphylococcus aureus. The predominant anaerobes were anaerobic gram-positive cocci, Fusobacterium nucleatum, and Bacteroides fragilis. Replacement of one pathogen by another occurred frequently. Tracheobronchitis occurred in 24 patients, all of whom had episodes of pneumonia. The data suggest that anaerobic bacteria are a part of the bacterial flora in colonization, tracheobronchitis, and pneumonia in patients with tracheostomy and prolonged intubation.