ABSTRACT The role of Acinetobacter nosocomialis and Acinetobacter pittii , which belong to the A. calcoaceticus - A. baumannii complex, in hospital-acquired infections is increasingly recognized. Here we describe a retrospective cohort study of hospital-acquired A. calcoaceticus - A. baumannii complex infections at a university hospital in Thailand. A total of 222 unique cases were identified between January 2010 and December 2011. The genomospecies of the A. calcoaceticus - A. baumannii complex isolates were classified as follows: A. baumannii , 197 (89%); A. nosocomialis , 18 (8%); and A. pittii , 7 (3%). All A. nosocomialis and A. pittii isolates were susceptible to imipenem and meropenem. The patients infected with A. nosocomialis and A. pittii had lower 30-day mortality than those infected with carbapenem-susceptible A. baumannii ( P = 0.025) and carbapenem-resistant A. baumannii ( P = 0.013). The factors influencing 30-day mortality were infection with non- baumannii A. calcoaceticus - A. baumannii complex (hazard ratio [HR], 0.12; 95% confidence interval [CI], 0.03 to 0.51; P = 0.004), infection with carbapenem-resistant A. baumannii (HR, 1.57; 95% CI, 0.89 to 2.79; P = 0.105), appropriate empirical antimicrobial therapy (HR, 0.38; 95% CI, 0.23 to 0.61; P < 0.001), and higher acute physiology and chronic health evaluation II (APACHE II) score (HR, 1.15; 95% CI, 1.10 to 1.19; P < 0.001). In Galleria mellonella assays, the survival rates were significantly higher for the larvae infected with A. nosocomialis or A. pittii than for those infected with either carbapenem-susceptible A. baumannii or carbapenem-resistant A. baumannii , but no differences in survival rates were observed between carbapenem-susceptible A. baumannii and carbapenem-resistant A. baumannii . These findings suggest intrinsic differences in virulence between non- baumannii A. calcoaceticus - A. baumannii complex species and A. baumannii but not between carbapenem-susceptible and resistant A. baumannii .