Over a 12-year period (1989 to 2000), 159 Klebsiella pneumoniae isolates harboring extended-spectrum beta-lactamases (ESBLs) (4.8% of the total number of K. pneumoniae isolates obtained) were recovered from 58 patients, who were mainly hospitalized in intensive care and surgery units. For 62 representative isolates from 58 patients, 31 clonal types harboring TEM-4 (n = 5), SHV-2 (n = 7), SHV2a (n = 4), SHV-5 (n = 1), CTX-M-10 (n = 13), or CTX-M-9 (n = 1) beta-lactamases were identified by pulsed-field gel electrophoresis. This is the first report to document the presence of the CTX-M-10 or the CTX-M-9 beta-lactamase in K. pneumoniae. These beta-lactamases were previously identified in Escherichia coli isolates from Spain. Only two of five K. pneumoniae TEM-4 clones caused more than a single case of infection, with one of them spreading for 9 months. A single plasmid was detected among these TEM-4 clones. Only two of seven K. pneumoniae clones containing SHV-2 and three of four strains harboring SHV-2a were detected in more than one case of infection. Plasmids encoding SHV-2 or SHV-2a were unrelated. Four of 13 K. pneumoniae CTX-M-10 clones were found in more than one patient, with two of them recovered 2 and 5 years apart. As in the case of the SHV-2 isolates, we were unable to document a common transmissible genetic element that could explain the polyclonal structure of our isolates. Nevertheless, the spread of a single gene may be suggested by the presence of a conserved set of noncoding polymorphisms in the sequences. Most ESBL-producing K. pneumoniae clones were ephemeral, being poorly selected and maintained in the hospital setting, but the genes encoding ESBL persisted successfully over the years that the strains were recovered, probably as a minority gene population in the hospital metagenome.