The co-existence of carbapenemase, 16S rRNA methylase and mutated quinolone resistance-determining regions (QRDRs) can cause serious difficulty in treating infections with multidrug-resistant Acinetobacter baumannii. In this study, we aimed to determine the mechanisms of imipenem, amikacin and ciprofloxacin resistance in A. baumannii isolates with resistance to these antibiotics. A total of 31 non-duplicate isolates of amikacin- and ciprofloxacin-resistant Acinetobacter isolates were identified from April to August 2010 from a single hospital in South Korea. To assess the clonal relatedness of the 31 Acinetobacter isolates, multilocus sequence typing, network phylogenetic analysis and enterobacterial repetitive intergenic consensus-PCR were utilized. Detection of OXA-type carbapenemase and 16S rRNA methylase was conducted using a multiplex PCR assay. The QRDRs of the gyrA and parC genes were amplified and sequenced. The result showed that 30/31 isolates harboured the blaOXA-23-like carbapenemase, which made them resistant to imipenem (MICs ≥16 µg ml(-1)). Twenty-eight of the 31 isolates were found to possess armA, a 16S rRNA methylase gene, and showed resistance to amikacin, arbekacin, gentamicin and tobramycin (MICs >256 µg ml(-1)). All of the isolates were determined to carry QRDR mutations in both gyrA and parC: a Ser83Leu substitution in gyrA and a Ser80Leu substitution in parC, causing a ciprofloxacin MIC ≥64 µg ml(-1). In conclusion, A. baumannii with co-existence of carbapenemase, 16S rRNA methylase and mutated QRDRs are extremely prevalent in South Korea, which may cause serious problems in the treatment of A. baumannii infections using carbapenem, amikacin and ciprofloxacin.