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Summary Objective: To develop a simple analytical method for monitoring serum and urine concentrations of mycophenolic acid (MPA), an active metabolic constituent of the immunosuppressive pro-drug mycophenolate mofetil, and its glucuronide. Methods: Serum samples were prepared by solid-phase extraction (SPE), while urine samples were simply diluted with water. Serum was added to an SPE cartridge, then washed twice with 5% methanol solution. The analytes were eluted with methanol containing benzoic acid as internal standard for mycophenolic acid glucuronide (MPAG). The resultant eluate was directly injected into a high-performance liquid chromatograph (HPLC) to determine MPAG. For the assay of MPA, the remaining eluate was dried under nitrogen and resolved in a mixture of acetonitrile and 20 mm phosphate buffer (pH 3&bdot;0). Results: The present methods were reproducible and accurate based on the intra- and inter-assay, and had detection limits of 0&bdot;225 μg/mL for MPA and 9&bdot;0 μg/mL for MPAG. The present methods enabled us to monitor the time course of changes in the concentrations of MPA and MPAG in serum and urine in a patient with a renal transplant during 12 h after ingestion of mycophenolate mofetil. Conclusion: The HPLC method described should be useful for the routine monitoring of serum and urine concentrations of MPA and MPAG during immunosuppressive medication for renal transplantation. [ABSTRACT FROM AUTHOR]