Over a 12-month period, 27% of patients in a new ICU grew bacterial pathogens from sputum or tracheal cultures. The commonest isolates werePseudomonas aeruginosaandKlebsiellaspecies. Endotracheal intubation, the length of time intubated, and antimicrobial therapy all predisposed to the isolation of organisms from sputum.No patient developed a gram-negative pneumonia, and there was no case of septicemia associated with a positive sputum culture. The presence of epithelial or pus cells in sputum was unrelated to the culture results. It was concluded that the growth of colonic bacteria from sputum or tracheal aspirates was of little prognostic or clinical significance.No significant common environmental site or cross-infection pathway was identified: sinks were contaminated by patients rather than vice versa. Most sputum isolates were probably endogenous in origin.