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Background Multidrug-resistant Acinetobacter baumannii (MDRAB) is an important and emerging hospital-acquired pathogen worldwide.This study was conducted to identify the sources of MDRAB and its role in respiratory tract colonization and nosocomial pneumonia in intensive care unit (ICU) patients.Methods We conducted a prospective active surveillance study of MDRAB in three ICUs at a Chinese Hospital from April to August 2011,to identify the sources of MDRAB and its role in respiratory tract colonization and nosocomial pneumonia.Results One hundred and fourteen (13.0％) MDRAB isolates were detected from 876 specimens,with a sensitivity of 11.6％ (55/474) in screening of the pharyngeal and tracheal swabs,and 14.7％ (59/402) of the sputum/endotracheal aspirates.MDRAB colonization/infection was found in 34 (26.8％) of 127 patients,including 16 (12.6％) cases of pure colonization and 18 (14.2％) cases of pneumonia (two pre-ICU-acquired cases of pneumonia and 16 ICU-acquired cases of pneumonia).Previous respiratory tract MDRAB colonization was found in 22 (17.3％) patients:eight (6.3％)were pre-ICU-acquired colonization and 14 (11.0％) ICU-acquired colonization.Of eight pre-ICU-colonized patients,five were transferred from other wards or hospitals with hospitalization ＞72 hours,and three came from the community with no previous hospitalization.Overall,6/22 colonized patients presented with secondary pneumonia； only two (9.1％) colonized MDRAB strains were associated with secondary infections.Respiratory tract MDRAB colonization had no significant relationship with nosocomial pneumonia (P=-0.725).In addition,acute respiratory failure,mechanical ventilation,renal failure,and prior carbapenem use were risk factors for MDRAB colonization/infection.Conclusions A high proportion of cases of MDRAB colonization/infection in ICU patients were detected through screening cultures.About one-third were acquired from general wards and the community before ICU admission.The low incidence of MDRAB colonization-related pneumonia questions the appropriateness of targeted antibiotic therapy.