A prospective randomized comparison of blood concentration as well as manifestations of ototoxicity and nephrotoxicity in groups with conventional and individualized administration of gentamicin was carried out in a total of 106 hospitalized patients. Therapeutic serum concentrations (peak 4-9 mg/L, though less than 1 mg/L) were achieved in 50% of the patients of conventional group and in all the patients of individualized group. Incidence of auditory and renal toxic reactions was high in the conventional group, being 27% and 30% respectively, while in the individualized group, the corresponding figures were 2.8% and 0%. Therefore, individualized administration of gentamicin based on data from therapeutic drug monitoring can markedly decrease the toxic effect and increase the therapeutic efficacy. It is worthwhile to use this kind of administration clinically.