Carbapenems are among the drugs of choice for treating nosocomial infections due to multidrug-resistant Acinetobacter baumannii strains, but their efficacy is compromised by carbapenemases like OXA-23. Previously, blaOXA-23 associated with insertion sequence ISAba4 was reported in two French A. baumannii isolates. Here, we describe the first ISAba4 blaOXA-23-expressing A. baumannii isolates in Belgium. In July 2007, a 69-year-old Belgian resident transferred from Morocco to Belgium with pneumonia and cardiac failure had respiratory secretions yielding carbapenem-resistant A. baumannii strain GIL1; the patient died despite colistin treatment. GIL1 was identified via API 32GN and intrinsic blaoxa-51/69-like gene, and showed resistance to all β-lactams (including imipenem/meropenem, MIC 32 μg/ml), sulbactam, ciprofloxacin, amikacin; borderline susceptibility to tigecycline and rifampin; and sole susceptibility to colistin. PCR sequencing revealed GIL1 harbored blaOXA-23 (associated with ISAba4), blaOXA-69, and a novel blaADC allele (blaADC-39, GenBank EU652244, with two amino acid changes from AYE blaAmpC). Isoelectric focusing confirmed OXA-23 and ADC-39 expression. blaOXA-23 was encoded on a 25-kb plasmid transferable to A. baumannii CIP7010 by electroporation. Six additional ICU patients had ISAba4-blaOXA-23 isolates indistinguishable by pulsed-field gel electrophoresis (PFGE), indicating clonal diffusion. GIL1 also shared identical PFGE patterns with a French ISAba4-blaOXA-23 isolate. This highlights the need for epidemic surveys to estimate ISAba4-OXA-23 producer prevalence and further investigation of ISAba4's fitness advantage over ISAba1 in A. baumannii.