Between December 1994 and April 1995, a nosocomial outbreak caused by a multi-resistant Acinetobacter baumannii, occurred on a surgical ward in our hospital. The organism was isolated from 13 patients, eight of whom were infected whereas the others were colonized. Twelve isolates were compared by cell envelope protein electrophoretic profiles and AFLP, a recently described DNA fingerprinting method. Both methods indicated that this outbreak was caused by spread of a single strain, which was identified as A. baumannii by amplified ribosomal DNA fingerprinting (ARDRA). A case-control comparison was performed to identify risk factors associated with nosocomial acquisition of A. baumannii. Risk factors for cross-colonization were length of stay, surgery, wounds and treatment with broad-spectrum antibiotics. Cross-infection with A. baumannii among patients occurred despite implementation of stringent infection control measures. The outbreak was controlled after temporary closure of the surgical ward for disinfection purposes. Patients admitted on a general surgical ward colonized or infected with multi-resistant A. baumannii strains should alert the hospital infection control team, and prompt implementation of strict infection prevention measures to prevent further spread is advised.