The emergence of carbapenem resistance in Enterobacteriaceae represents a major public health concern. We investigated ertapenem-resistant clinical isolates of Klebsiella spp. and Enterobacter spp. referred to the UK's national reference laboratory for antibiotic resistance. Minimum inhibitory concentrations (MICs) were determined and interpreted according to British Society for Antimicrobial Chemotherapy guidelines. Genes for carbapenemases and CTX-M extended-spectrum beta-lactamases (ESBLs) were sought by polymerase chain reaction, and imipenem hydrolysis was determined by spectrophotometry with crude extracts. From June 2004 to April 2006, 95 Klebsiella spp. and 76 Enterobacter spp. isolates resistant to ertapenem (MICs >2mg/L) were received, 40% of which (38 Klebsiella spp. and 30 Enterobacter spp.) were highly resistant to ertapenem (MICs >16mg/L). Imipenem and meropenem were active (geometric mean MICs <2mg/L) against most isolates with low-level ertapenem resistance but were less active against highly ertapenem-resistant isolates. Only one ertapenem-resistant isolate produced a defined carbapenemase, a Klebsiella pneumoniae with IMP-1 enzyme; one Enterobacter sp. also hydrolysed imipenem, but its carbapenemase remains to be identified. Geometric mean MICs of ertapenem for the collection were reduced five-fold by clavulanic acid for Klebsiella spp. compared with eight-fold by cloxacillin for Enterobacter spp. This study highlights the fact that ertapenem resistance is being detected in Klebsiella spp. and Enterobacter spp. in the UK, but that it is rarely mediated by true carbapenemases. Rather, it probably results from combinations of a beta-lactamase - often a CTX-M ESBL in Klebsiella spp. or an AmpC enzyme in Enterobacter spp. - plus impermeability and/or increased efflux. Imipenem and meropenem often remain moderately active against isolates with low-level ertapenem resistance.