A multiresistant clone of Acinetobacter baumannii was identified in 24 hospitals in the UK, predominantly in the London area, over a period of three years. Isolates were characterized by distinctive ApaI macrorestriction profiles, as resolved by pulsed-field gel electrophoresis (PFGE), which all clustered within 80% similarity using a 1% band position tolerance setting. The first isolates identified were received by the reference laboratories in April 2000, and by June 2003, a total of 375 isolates with similar PFGE profiles from 310 patients from 24 hospitals had been received. The isolates originated mainly from sputum and wound specimens, with the majority from patients in intensive care units. Amplified fragment length polymorphism analysis of a subset of isolates showed that they clustered closely, supporting the PFGE results. All the isolates tested were highly resistant to ampicillin, piperacillin, piperacillin/tazobactam, ceftazidime, cefotaxime, gentamicin and ciprofloxacin, and most isolates were carbapenem resistant. Amikacin sensitivity varied from susceptible [minimum inhibitory concentration (MIC) <or= 4 mg/L] to highly resistant (MIC >256 mg/L).