Gentamicin is useful to the Gram negative bacterial infection, but its nephrotoxicity is a serious problem and the incidence is probably increasing. The toxicity of gentamicin to the kidney is site-specific to the proximal tubule. In this study, we measured daily peak and trough level of gentamicin, serum creatinine, serum $Beta_2$-microglobulin and 24-hr urine $Beta_2$-microglobulin in 10 gentamicin treated patients. All the patients had their peak levels of gentamicin in the safe therapeutic range, and their trough level showed no evidence of gentamicin accumulation. There was no patient who showed his daily serum creatinine and $Beta_2$-microglobulin rise significantly. But 24-hour urine $Beta_2$-microglobulin showed significant rise from basal level (mean $5.8{pm}1.62{ imes}$) on the 5 th day of gentamicin treatment. Thus, serial monitoring of proximal tubular function with urinary $Beta_2$-microglobulin excretion has potential value in the assessment of insults of gentamicin to this site. But clinical significance of raised urinary $Beta_2$-microglobulin excretion in relation to the serum creatinine should be further studied.