BACKGROUND: In recent years, the incidence of extended-spectrum beta-lactamase (ESBL) producing Klebsiella has been steadily increased, and the newer species K. planticola and K terrigena, formerly regarded as nonpathogen, have been reported with astonishing frequency from human infectious processes by some investigators. The aim of this study is to elucidate the isolation rate and antimicrobial susceptibility of recent clinical Klebsiella isolates. METHOD: For the clinical Klebsiella isolates during the period of June 1999 to May 2000, isolation frequency of Klebsiella species by specimen, departments, age, and sex were analyzed. And antimicrobial susceptibilities were also analyzed. RESULT: Isolation rate of Klebsiella in order of decreasing frequency were K. pneumoniae (74:7%), K. oxytoca (12.1%), K. ozaenae(1.7%), K. planticola(1.0%), K. terngena(0.9%), and K, ornithinolytica (0.7%), respectively. K. rhinoscleromatis was not isolated. Compared with outpatients, increase of resistance rates of inpatients's Klebsiella isolates were 10% in ciprofloxacin, 15% in cefoperazone/sulbactam, and the others were ranged from 24% to 31%. Isolation rate of ESBL producing K. pneumoniae by double disk (DD) synergy test was 41%, and detection rates by antimicrobial agents were as follows: cefotaxime (95%), aztreonam (58%), and ceftriaxone (37%). Antimicrobial susceptibility rate with the exception of ampicillin and imipenem decreased from the range of 81%-96% on admission day to 29-62% after one week on admission. CONCLUSION: The isolation rates of K. planticola and K. terrigena were less than 1%. The proportion of ESBL producing K. pneumoniae was 41 %. And the vast majority of multidrug resistant Klebsiella including ESBL producing strains are acquired by hospitalization.