BACKGROUND: Acinetobacter baumannii is an important nosocomial pathogen being reported with increasing frequency in outbreaks during the past decade. This prospective study was initiated to identify risk factors for the nosocomial acquisition of A. baumannii in patients admitted into a medical intensive care unit (MICU). METHODS: Nasal, rectal and skin swabs were obtained from patients within the 48 hours of admission to the MICU and weekly thereafter during the study period and the final swabs were taken at the time of discharge. If A. baumannii was isolated in the follow up surveillance or from clinical specimen, further culture was not done. Isolates were identified by using the morphology of the colonies in selective media, MicroScan and the ability to grow at 44degrees C. Risk factors of the patients with or without nosocomial acquisition of A. baumannii were compared. RESULTS: Sixty one of the 438 patients admitted to the MICU during the study periods were enrolled. Acquisition of A. baumannii was found in 28 (45.9%) of the 61 patients. The application (89.3% vs. 54.5%, P<0.01) and duration of mechanical ventilation (11.9+/-11.0 day vs. 7.9+/-11.0 day, P<0.05), and the useof nasogastric tubes (100% vs. 78.8%, P=0.01) were the significant risk factors associated with the acquisition of A. baumannii. The proportion of the patients whose APACHE III scores were more than 40 was higher in the acquired group compared to the non-acquired group (92.9% vs. 69.7%, P<0.05). The length of stay in MICU of the acquisition group was longer than those without acquisition (27.0+/-21.0 day vs. 18.7+/-17.0 day, P<0.05). Overall mortality of the patients with acquisition of A. baumannii was higher than in those without acquisition (53.6% vs. 27.3%, P<0.05). However, only the severity of illness evaluated by the APACHE III score (P<0.05) was retained as an independent risk factor for high mortality (odds ratio 1.05, 95% confidence interval 1.01~1.08). Most of A. baumannii showed multi-resistance to antimicrobial agents except imipenem. CONCLUSION: In our study, the risk of the nosocomial acquisition of A. baumannii was associated with the application of mechanical ventilation or nasogastric tube, the severity of illness, and prolonged MICU stay. Acquisition of A. baumannii was not associated with excess mortality, but the severity of illness evaluated by the APACHE III score was retained as an independent risk factor for high mortality.