We have developed a new fully automated method for mycophenolic (MPA) acid quantification in plasma to optimize therapeutic drug monitoring of tranplant patients. This method involved solid-phase extraction on disposable extraction cartridges and reversed-phase high-performance liquid chromatography with diode array detection. Solid-phase extraction was performed automatically by an automated sample with extraction catridges system. After washing, MPA was eluted from the cartridge onto a Chromolith RP-18e column. MPA and the internal standard were detected at 306 nm. The retention time of MPA was 6.3 minutes. The developed method was linear from 0.2 to 20 μg/mL. The limit of quantification was 0.2 μg/mL. The method showed a good precision with intraday and interday variation coefficient less than 6%. The intraday accuracy ranged from 97.6% to 100.4% and the interday accuracy varied from 97.1% to 100.8%. The extraction efficiency was greater than 90%. This method is simple and shows a good specificity with respect to commonly co-prescripted drugs.