A case-control study was conducted to determine risk factors for acquisition of an epidemic strain of Acinetobacter baumannii in an intensive care unit. The epidemic strain was identified by pulsed-field gel electrophoresis and was isolated from tracheal secretions in 13 (87%) of 15 patients. In a logistic regression analysis, presence of a tracheostomy was an independent risk for Acinetobacter sp acquisition (odds ratio, 421; 95% confidence interval, 13.8-12925; P =.001) and the strength of the association was inversely related to the duration of mechanical ventilation. The outbreak coincided with the introduction of a policy of early percutaneous tracheostomy in the intensive care unit and probably resulted from inadequate infection control practices during respiratory care. No environmental reservoir was found. Institution of contact precautions, enhanced handwashing, and staff education was associated with resolution of the outbreak.