Multidrug-resistant Acinetobacter baumannii has been recently recognized as a major cause of nosocomial infections. During an outbreak of a multidrug-resistant strain, all isolates of A. baumannii were analyzed by pulsed-field gel electrophoresis, and a retrospective review was conducted to identify risk factors for acquisition. Of the 65 patients, the unique outbreak clone (clone I) was isolated from 43 (group A), whereas 19 different strains were grown from 22 other patients (group B). Clone I was predominantly isolated from pleuropulmonary sources (63%; P < 0.0001). Imipenem resistance was nearly universal in clone I, and amikacin resistance was common. In univariate analysis, admission to the medical/surgical intensive care unit and use of mechanical ventilation and fluoroquinolone antibiotics were associated with clone I. Only admission to the medical/surgical intensive care unit was significant by multivariate analysis. The institution of an infection control program controlled but did not eliminate the outbreak.