Introduction. Tacrolimus has immunosuppressive effects in vitro through the inhibition of mixed lymphocyte reactivity and the generation of cytotoxic T cells. There is a wide variety of laboratory methods for monitoring of immunosuppressive drugs. There is a growing clinical needfor a new evaluation of these methods. Material and Method. A total of 17 transplant patients were retrospectively analyzed. This study aimed to evaluate the concordance between ELISA and LC-MS/MS methods for measurement of tacrolimus serum trough levels. Result. Mean tacrolimus blood concentrations in these patients ranged from 2.43-10.07 ng/ml for the ELISA and 3.1-7.6 ng/ml for the LC-MS/MS. We found a concordance between ELISA and LC-MS/MS but the inter-subject variance coefficient for the ELISA method was higher than inter-subject variance coefficient for LC-MS/MS. Conclusion. The results point out the advantages of LC-MS/MS over ELISA in monitoring tacrolimus levels in pediatric kidney transplanted patients. The lower inter-subject variance of LC-MS/MS could result in fewer dose changes and an overall better immunosuppression control.