OBJECTIVE: To understand the mechanisms and epidemiology of resistance to oxyiminocephalosporins in Escherichia coli over a 2-year period in a French hospital. METHODS: Forty-four strains, resistant or intermediately resistant to one of the oxyiminocephalosporins or aztreonam, were collected from 35 patients. MIC determinations were carried out for the 44 isolates using a panel of beta-lactam antibiotics, and characterization of the beta-lactamases they produced by isoelectric focusing and catalytic activity measurement. Extended-spectrum beta-lactamase production was studied by use of the double disk diffusion test. Conjugation experiments were used to search for plasmidic cephalosporinase. An epidemiologic study was then performed, by use of molecular typing of the strains with an ERIC-PCR method and a case-control analysis. RESULTS: Less than 1% of all the E. coli isolates at our hospital showed decreased susceptibility to oxyiminocephalosporins. Only three of the 44 isolates showed synergy between clavulanate and a third-generation cephalosporin and produced an extended-spectrum beta-lactamase. For the other strains, a beta-lactamase with a highly basic isoelectric point was detected. Spectrophotometric measures confirmed that most of these isolates were AmpC hyper-producers. No plasmidic cephalosporinase could be detected by conjugation experiments. Molecular typing showed all isolates to be different, except for two strains isolated in two patients of the same hospital unit, and for the repeated isolates of some patients. When 20 case patients were compared to 40 randomly selected control patients, prior receipt of an antimicrobial and more specifically of a beta-lactam agent was significantly associated with case patients. CONCLUSIONS: Although it appears to be very rare, the resistance to broad-spectrum cephalosporins needs our attention, because of the high frequency of E. coli infections and beta-lactam use in their treatment.