The incidence of extended-spectrum beta-lactamases (ESBL)-producing Klebsiella pneumoniae and Escherichia coli has been increasing rapidly, and they are probably even more prevalent than is currently recognized because of difficulties in their detection by the clinical microbiology laboratory. In addition, several outbreaks associated with these multiresistant strains have been reported. In the present study we evaluated 30 clinical isolates (27 K. pneumoniae from 11 hospitals and three E. coli from three hospitals) that were resistant or intermediately susceptible to ceftazidime and/or cefuroxime. The main objective of the study was to evaluate the intra- and interhospital dissemination of ESBL-producing strains. The isolates were tested for susceptibility to ceftazidime, cefuroxime, gentamicin, and ofloxacin, and the ability of various susceptibility testing methods to detect resistance to these beta-lactams was evaluated. The production of ESBL was assessed by the disk approximation synergy test, and typing was performed by pulsed-field gel electrophoresis (PFGE) of chromosomal DNA. ESBL production was demonstrated in 15 K. pneumoniae (from seven hospitals) and in one E. coli strain. Most ESBL-producing isolates demonstrated cross resistance with gentamicin and ofloxacin. Chromosomal DNA analysis by PFGE exhibited a great genomic variability among ESBL-producing isolates. Our results also indicated the occurrence of both intra- and interhospital dissemination of these multiresistant strains.