Eight pairs of Escherichia coli isolates with various carbapenem susceptibilities from 8 patients were prospectively collected to study the development of resistance. All carbapenem-resistant E. coli isolates were resistant to all tested ss-lactams antibiotics except tigecycline. Identical pulsed-field gel electrophoresis (PFGE) patterns were found in carbapenem-susceptible and -resistant isolates but different PFGE patterns occurred among patients. A CMY-2 ss-lactamase was found in all E. coli isolates. No previously reported carbapenemase genes were detected. Examination of outer membrane protein (OMP) profiles revealed that OmpA was not found in all isolates, while OmpC and OmpF were lost in carbapenem-resistant isolates. Loss of both OmpC and OmpF represents the major mechanism of the development of carbapenem resistance in those patients with CMY-2-producing E. coli infections.