BACKGROUND: Staphylococcus epidermidis is a leading cause of nosocomial infections, and resistance to methicillin is common in clinical isolates. The distribution of oxacillin MIC for S. epidermidis is not clearly bimodal and it is suspected that the sensitivities for detection of oxacillin resistance by standard susceptibility assays with National Committee for Clinical Laboratory Standards (NCCLS) MIC interpretive criteria (< OR =2 g/mL, > OR =4 g/mL) in S. epidermidis strains are lower than that in Staphylococcus aureus strains. To evaluate the relationship between MIC results and true methicillin resistance, we examined the oxacillin MICs and methicillin MICs by agar dilution and detection of mecA gene by PCR for 41 S. epidermidis strains. METHODS: A total of 41 S. epidermidis strains were examined antimicrobial susceptibility test by VITEK system with GPS-AA card, oxacillin MICs and methicillin MICs by agar dilution and detection of mecA gene by PCR. RESULTS: In antimicrobial susceptibility test by VITEK system with GPS-AA card, 24 strains (58.5%) showed oxacillin resistance. 13 strains (31.7%) required MICs of > OR =4 g/mL in oxacillin MIC test and 19 strains (46.3%) required MICs of > OR =16 g/mL in methicillin MIC test. But 27 strains (65.9%) were mecA positive. One of 15 strains that required oxacillin MICs of < OR =0.5 g/mL, all 3 strains that required oxacillin MICs of 1 g/mL and all 10 strains that required oxacillin MICs of 2 g/mL were mecA positive. CONCLUSIONS: It is suspected that NCCLS MIC interpretive criteria underestimate methicillin resistance among S. epidermidis strains and the PCR method is a reliable reference method.