Aims The present study was conducted to investigate the mechanism of carbapenem resistance and the molecular epidemiology of carbapenem-resistant Acinetobacter baumannii (CRAB) isolates collected from two nearby hospitals in Tehran, Iran. Methods and Results A total of 180 CRAB isolates were studied. Antimicrobial susceptibility testing was performed using disk diffusion and Epsilometer tests. The detection of OXA-23, -24 and -58 was implemented for all isolates using polymerase chain reaction. Subsequently, isolates harbouring OXA-24 and -58 were investigated for the presence of resistance determinants of Ambler class A, metallo-beta-lactamases (MBLs), and carbapenem-hydrolysing class D beta-lactamases, ISAba1, and the genetic relatedness between them was analysed using pulsed-field gel electrophoresis (PFGE). All isolates were found to be resistant to imipenem with a MIC of >= 8 mu g ml(-1) and were susceptible to colistin with a MIC of <= 1 center dot 5 mu g ml(-1). Sixty percent of the isolates had OXA-23. OXA-24 and -58 were detected in 31 of 180 CRAB isolates. These chosen isolates were devoid of MBLs and bla(SHV), bla(C)(TX-M), bla(VEB )ESBL genes. The PER determinant was detected in 38% of isolates as the most common extended spectrum beta-lactamases (ESBLs). Of these isolates, 51 center dot 6% had OXA-23, and ISAba1 was found to be upstream of OXA-23 and OXA-51 in 16 and 8 isolates, respectively. The band patterns produced by PFGE showed nine clonal pulsotypes distributed between the two hospitals. Conclusion The findings showed that the refractory CRAB isolates were transmitted intra- and inter-hospital, particularly in the ICU due to shortcomings in infection control surveillance. Significance and Impact of the Study Carbapenem resistance is a substantial threat in the treatment of infections caused by A. baumannii due to limitations in the therapeutic options.