BACKGROUND: Liquid chromatography-tandem mass spectrometry (LC-MS/MS) are used increasingly for tacrolimus monitoring. However, there are still variability of results due to home-brew reagents, so which cannot be warrantable data. We evaluated the analytical performance and clinical usefulness of a newly introduced MassTrak LC/MS/MS Tacrolimus kit (Waters Corporation, USA). METHODS: The performance of LC-MS/MS for determination of tacrolimus concentration were analyzed using patient samples and MassTrak LC/MS/MS Tacrolimus kit including calibrators, quality controls, internal standard, column and neat solution with respect to linearity, precision, lower limit of detection, lower limit of quantitation, sample carryover and comparison according to CLSI guidelines. The LC-MS/MS using home-brew reagents were performed for comparison test. RESULTS: The LC-MS/MS using MassTrak LC/MS/MS Tacrolimus kit showed a good linearity (R2> or =0.997) and precision (CV< 8%). Assigned LLOD (0.4 ng/mL) and LLOQ (0.8 ng/mL) were validated and carryover was estimated 0.5%. The system correlated well with the LC-MS/MS using home-brew reagents (R> or =0.974). CONCLUSIONS: LC-MS/MS using MassTrak LC/MS/MS Tacrolimus kit for determination of tacrolimus concentration showed good performance for linearity, precision, LLOD, LLOQ, carryover and comparison. Introduction of MassTrak LC/MS/MS Tacrolimus kit could be warranted results by manufacturer and useful for management of quality control.