Abstract : Gentamicin is an antibiotic with a narrow therapeutic index, therefore to determine the dosage must be individualized to made concentration of drug in the blood reaches the level of effectiveness and does not cause toxicity and prevents drug resistance. This research aims to see the accuracy of gentamicin concentration from the doses given. This type of research is a descriptive observational study with a retrospective approach to 42 medical record samples that received gentamicin using purposive sampling technique in RSUD Arifin Achmad. The results of the study showed that the percentage of accuracy of the Cssave concentration of 97.62% indicated that the dose given was in the therapeutic range and 2.38% below the therapeutic range. Accuracy of concentration based on Cssmax, exact concentration of 14.28%, concentration of less than 82.86% and excess concentration of 2.86%. The accuracy of the Cssmin concentration is 100% the right concentration. So can be concluded that the dose of gentamicin given to pediatric patients is not yet reached its concentration in providing effectiveness, because dose given to the patient was not based on calculation of individual doses.