TEM- or SHV-type extended-spectrum beta-lactamases (ESBLs) are of clinical concern in Europe and the United States, whereas bacterial strains producing such types of ESBLs have not been reported in Japan. We report here two cases of infection due to Klebsiella pneumoniae resistant to extended-spectrum cephalosporins in Japan. A ceftadizime-resistant K. pneumoniae strain (minimum inhibitory concentration; 32 &mgr;g/ml) was isolated transiently from the sputum of an 87-year-old woman with acute myocardial infarction and pneumonia (patient 1). Ceftadizime-susceptible and -resistant (minimum inhibitory concentration; >/=8 &mgr;g/ml) K. pneumoniae strains were isolated over a month from the blood, ascites, and feces of a 44-year-old man after bone marrow transplantation for acute lymphoblastic leukemia (patient 2); this patient died of K. pneumoniae sepsis and peritonitis followed by multiple organ failure. These isolates produced penicillinase, which was inhibited by clavulanic acid. A polymerase chain reaction (PCR) study showed that both isolates carried the SHV or LEN genes, but not the TEM, Toho-1, and IMP-1 genes. The pulsed-field gel electrophoresis profile of the strain isolated from patient 1 was genetically distinguishable from the profiles of the strains isolated from patient 2. It appeared that mutation of the beta-lactamase gene may have occurred in the body of patient 2, since the genotypes of the ceftadizime-susceptible and -resistant isolates from this patient were identical. Another 12 strains of K. pneumoniae, isolated from other patients in the same wards during the period in which the K. pneumoniae strains were isolated from patients 1 and 2, did not produce ESBLs and showed different genotypes. The results suggest that these isolates of resistant K. pneumoniae did not spread by cross transmission in the hospital and that the two cases were sporadic. Surveillance of these types of resistant bacteria is necessary, since they may well be present in other hospitals in Japan. Although the organisms are suspected to produce SHV-type ESBLs or LEN-1 variant beta-lactamases, further studies are necessary to specify the resistance genes.