Efficacy of therapeutic drug monitoring (TDM) of vancomycin (VCM) was retrospectively investigated in 184 patients with methicillin-resistant Staphylococcus aureus (MRSA) infection. The incidence of nephrotoxicity was compared between the patients who received TDM practice (TDM group, n=73) and did not (non-TDM group, n=111). Creatinine clearance (CLcr) values decreased significantly after the VCM therapy in the non-TDM group (p<0.05). The patients with MRSA bacteremia or pneumonia were classified into two groups according to peak concentrations of VCM: above 25 μg/ml (Group A: n=29) and below (Group B: n=24). Mean duration of VCM therapy (14.1 d) in Group A was significantly shorter than that (27.0 d) in Group B. Mean cumulative total VCM doses (13.3 g) in Group A was significantly less than that (25.0 g) in Group B. These results indicate that monitoring peak concentration is essential to obtain better clinical effects for VCM therapy, and that the peak concentration above 25 μg/ml is more effective.