INTRODUCTION:During 2009, an outbreak of multidrug-resistant Acinetobacter baumannii (MDR A. baumannii) infections was detected in a 27-bed intensive care unit, resulting in 25 cases being infected. METHODS:A matched case-control study was conducted to identify risk factors for infection. The colonization pressure, or the proportion of other patients colonized by MDR A. baumannii, was estimated. TISS-28 and Omega scores of each patient were calculated to evaluate nursing work requirements. Conditional logistic regression analyses were carried out. RESULTS:Breakdowns in hand washing and glove use were observed. Infected patients (cases) were more likely than paired controls to have had longer exposure to invasive devices and antimicrobial treatment. The independent risk factors identified by the multivariate analysis were, mechanical ventilation [odds ratio (OR)=1.03; 95% confidence interval (CI), 1.01-1.05; P=.01], and exposure to an infected or colonized patient [OR=1.7; 95%CI, 1.1-2.6; P=.02). A combined infection control strategy was implemented, including strict compliance with isolation precautions, grouping of patients, reinforcing cleaning and disinfection of surfaces, and a decrease in work load. Subsequently, a sharp reduction in the incidence MDR A. baumannii infections was shown. Therapeutic activity scores were significantly higher for cases than for controls. CONCLUSION:The results suggest patient-to-patient transmission of MDR A. baumannii. Reinforcement of specific procedures and work load adjustment were essential to eradicate this outbreak.