Eleven genotypically related Klebsiella pneumoniae isolates were obtained from 11 patients. All isolates were resistant to third-generation cephalosporins due to the production of SHV-2a extended-spectrum β-lactamase. Comparison of the outer membrane protein profiles revealed one isolate that lacked porins. This porin-deficient isolate was also resistant to cefoxitin (MIC 128μgml −1 ) and moxalactam (MIC 64μgml −1 ) and had elevated MIC of meropenem (2μgml −1 ) when compared to porin-expressing isolates (2–8, 4 and <0.06–0.125μgml −1 , respectively). Higher MICs, associated with loss of porins in outer membrane, were also observed for cefotaxime (4–8-fold), cefepime (>2–16-fold), ciprofloxacin (4–16-fold), imipenem and aztreonam (2–16-fold), but there was no significant difference among MICs of ceftazidime. The porin-deficient mutant was probably selected in vivo during ofloxacin therapy.