BACKGROUND: Increasing numbers of resistant and multidrug resistant (MDR) isolates of Pseudomonas aeruginosa have become a worldwide problem. This report provides the trend of antimicrobial resistance, the proportions of MDR and metallo-beta-lactamase-producing isolates among clinical isolates of P. aeruginosa in Korea. MATERIALS AND METHODS: Clinical isolates of P. aeruginosa were collected from two representative reference laboratories during 2002-2004. Clinical information regarding specimens and type of hospital for isolates was investigated. Antimicrobial susceptibility against 11 antibiotics was tested by disk diffusion according to NCCLS criteria. MDR was assessed as resistance to > or =3 of the core drugs (ceftazidime, ciprofloxacin, gentamicin, imipenem and piperacillin). PCR assays and sequencing for detection of blaVIM-2 and blaIMP-1 gene were carried out. RESULTS: Of 1,748 P. aeruginosa isolates, 179 isolates were collected from primary care hospitals and 1,569 isolates were recovered from outpatients and inpatients in secondary care hospitals. From 2002 to 2004, rates of resistance to ceftazidime and imipenem increased from 10% to 12.3% and from 14.8% to 15.9%, respectively. Rates of resistance to amikacin (from 26.2% to 31.0%) and ciprofloxacin (from 35.6% to 46.2%) increased annually. In the period 2002-2004, decreasing of susceptibility to meropenem (from 83.4% to 76.8%) was observed, but meropenem was the most potent agent against P. aeruginosa isolates studied. During the 3-year period, MDR P. aeruginosa accounted for 26.4-33.5% of clinical isolates and the most common MDR phenotype was concurrent resistance to piperacillin, gentamicin and ciprofloxacin. The prevalence of VIM-2-producing isolates obviously increased from 1.7% in 2002 to 6.3% in 2004. CONCLUSIONS: These results suggested that MDR P. aeruginosa was already prevalent in one third of clinical isolates and VIM-2-producing P. aeruginosa isolates disseminated in non-tertiary care hospitals in Korea.