Objectives.To perform a molecular and epidemiologic investigation of multidrug-resistant (MDR)Acinetobacter baumanniiin an institution were polyclonal outbreaks have been observed and determine whether these polyclonal outbreaks had an endogenous origin or were caused by in-hospital patient-to-patient transmission.Design.Retrospective analysis of prospectively collected data.Setting.An epidemiologic and genotypic investigation of incident cases of MDRA. baumanniiinfection in an Israeli university tertiary care center.Patients.Hospitalized patients with MDRA. baumanniiisolated from clinical samples during a 13-week period, from April 15, 2003, through July 15, 2003.Intervention.All patients with new MDRA. baumanniiinfections were recruited, and isolates were typed using pulsed-field gel electrophoresis. Data on in-hospital movements and consultations were extracted from computerized databases. Quantification of transmission opportunities (TOPs), defined as encounters between an established carrier and a future carrier of MDRA. baumannii, and analysis of ward clusters were performed.Results.We studied 96 MDRA. baumanniiisolates, which belonged to 18 different pulsed-field gel electrophoresis clones. In 65% of cases, TOPs involving patients with the same clone were demonstrated, which is significantly greater than the number of TOPs involving patients with different clones (P= .01).Conclusion.Although outbreaks of MDRA. baumanniiinfection may be polyclonal, we believe that patient-to-patient transmission explains most cases of transmission. Polyclonal local outbreaks reflect several clonal outbreaks occurring simultaneously. The cause of polyclonal outbreaks ofA. baumanniiinfections clustered by ward and time remains to be explained.