In an Intensive Care Unit, three patients were found infected and two colonized with multiresistant Acinetobacter baumannii within a period of one week. To identify the outbreak source, two surveillance studies were performed concerning patients and the environment. Genotyping of isolates was performed by random amplification of polymorphic DNA analysis (RAPD). Environmental sampling failed to yield A. baumannii, with the exception of a single sample from a trunking. RAPD-fingerprinting yielded identical patterns for all patient isolates including the trunking isolate, thus confirming the suspected cluster. Since the strain from the trunking had a susceptibility pattern and a RAPD pattern identical to that of the strains isolated from the patients, we believe that this was the likely source of the outbreak. In conclusion, A. baumannii outbreaks may be quickly controlled by appropriate action of the hospital infection staff. RAPD-fingerprinting may provide a useful and rapid identification technique for the epidemiological investigation of a hospital outbreak.