During a three-month period in 1999, 25 strains of carbapenem resistant Acinetobacter baumannii were isolated from 12 of 170 hospitalized intensive care unit (ICU) patients, of which 16 were considered to be clinically significant. These strains were indistinguishable by biotyping and antibiograms, but genotyping was not performed. Appropriate antibiotic treatment, isolation precautions, and infection control education of the staff failed to halt the outbreak, Environmental contamination was therefore investigated, and A. baumannii was found out in 22 (39.3%) of 56 environmental samples obtained by swabbing. Different antibiotic sensitivity patterns were obtained in the majority of these isolates, but four (7.1%) of the strains were found to have the same sensitivity pattern as the strain causing the outbreak. As a result the ICU was closed, equipment and the environment cleaned, with hypochlorite and terminal disinfection carried out. No bacteria were grown on repeat environmental cultures. Environmental contamination has an important reservoir role in outbreaks of A. baumannii in ICUs and must be eradicated in order to overcome such outbreaks. (C) 2002 The Hospital Infection Society.