L-DOPA (L-3,4-dihydroxyphenylalanine) is commonly used in the treatment of Parkinson's disease. Monitoring the concentration of L-DOPA in human plasma will enable dose optimization, but is rarely performed because current quantification methods are tedious and time-consuming. In this study, a simple method for the determination of L-DOPA in the plasma of patients with Parkinson's disease was developed. A monolithic silica disk-packed spin column with a phenylboronate moiety, which forms stable anionic complexes with the cis-hydroxyl groups of L-DOPA, was used to extract L-DOPA from plasma with extraction recoveries exceeding 90%. The extracted L-DOPA was then separated on a reversed-phase column and detected electrochemically with a boron-doped diamond electrode. The method, which has a limit of detection of 10 fmol, was then successfully applied for the determination of L-DOPA in the plasma of healthy volunteers and patients with Parkinson's disease.