Carbapenem-resistant Acinetobacter spp. are being increasingly reported worldwide, including in South Korea, where we examined 144 representative isolates collected in a nationwide hospital survey in 2005. Metallo-ß-lactamases were detected in only 19.4% of isolates, none of which were Acinetobacter baumannii, whereas 74.3% of isolates (mostly A. baumannii) expressed blaOXA carbapenemase genes. Among the latter, 47 had blaOXA-23-like genes and 56 had upregulated blaOXA-51-like variants, including blaOXA-66, -83, -109 and -115; blaOXA-115 was a novel variant, detected in two isolates. blaOXA-72 (blaOXA-40-like) was detected in only a single Acinetobacter baylyi isolate, whilst three Acinetobacter calcoaceticus isolates had both blaVIM-2-like and blaOXA-58 genes. Pulsed-field gel electrophoresis (PFGE) suggested the spread of A. baumannii clones with OXA carbapenemases within and between hospitals. In conclusion, the recent increase in imipenem-resistant Acinetobacter spp. from South Korea is mostly due to OXA-type carbapenemases.