BACKGROUND: Accurate detection of extended spectrum beta-lactamase (ESBL) is important because ESBLproducing organisms may appear susceptible to oxyimino- beta-lactams in standard susceptibility tests, but are considered to be clinically resistant to these drugs. And continued monitoring of isolation trend of ESBL-producing organisms is essential for the guideline settlement of antibiotic usage and infection control program. METHODS: Disk diffusion test using the Clinical and Laboratory Standards Institute's ESBL phenotypic confirmatory test were performed on 5,511 clinical isolates of Escherichia coli, Klebsiella pneumoniae, Klebsiella oxytoca, and Proteus mirabilis during the recent six years (April 2001-March 2007). The ESBL producer was defined as an organism showing an increase in the zone diameter of > or =5 mm for either cefotaxime or ceftazidime with clavulanic acid versus that without clavulanic acid (CTC confirmatory test, CZC confirmatory test, respectively). RESULTS: The ESBL-positive rates were 34.8% in K. pneumoniae, 9.3% in K. oxytoca, 8.4% in E. coli, and 6.5% in P. mirabilis. Among the ESBL-positive organisms, the detection rates of ESBL CTC and CZC confirmatory tests were as follows: 91.3% vs 68.7% in K. pneumoniae, 96.3% vs 44.4% in K. oxytoca, 94.8% vs 45.4% in E. coli, and 100% vs 20% in P. mirabilis. ESBL-producing K. pneumoniae had shown a continuously increasing trend from 24.3% in 2001 to 46.4% in 2006. CONCLUSION: Both of the ESBL confirmatory tests should be simultaneously tested for the accurate detection of ESBL-producing K. pneumoniae, K. oxytoca, E. coli, and P. mirabilis. In addition, an active infection control approach is needed for ESBL-producing K. pneumoniae.