BACKGROUND: With increase of antibiotics use and invasive procedures, infections caused by multi-resistant Acinetobacter baumannii (MRAB) are increasing. Recently, we experienced the outbreak of- nosocomial infections caused by MRAB resistant to imipenem and cefoperazone/sulbactam in intensive care units (ICU) and general ward. We analyzed the clinical characteristics of the infected patients and antibiotic susceptibility of the organisms. And surveillance cultures and IRS-PCR were performed to find out the transmission route. METHODS: We collected data from physical examination and clinical records. We performed surveillance cultures of environment, patients not infected with MRAB in ICU, and hands of health care workers. RESULTS: Between November 1996 and June 1997, 49 strains of MRAB were isolated from the 26 patients hospitalized in Kangnam St. Mary' s Hospital. The lower respiratory infection (13 cases) was the most common infection and sputum was the most common sources (47.1%). All strains of MRAB showed the same genotype. In disk diffusion test, all strains were resistant to piperacillin, gentamicin, amikacin, ceftazidime, cefoperazone/sulbactam, aztreonam, imipencm, ciprofloxacin. From the surveillance cultures, the genotypically identical strains were isolated from ventilator Y-piece, the floor of ICU, and hands of health care workers. It suggested that this strain was transmitted through ventilatory device or hands of health care workers. We instructed all the health care workers to wash hands, to disinfect hospital environment completely. Since July 1997, no further case has occurred. CONCLUSIONS: Since A. baumannii could be transmitted through ventilatory devices and the hands of health care workers, it is important to wash hands and to disinfect hospital environment completely.