Elevated plasma homocysteine (Hcy) concentrations have been reported in L-dopa treated Parkinson's disease (PD) patients, suggesting that L-dopa treatment is an acquired cause of hyperhomocysteinemia. Aim of this study is to evaluate the effects of different antiparkinsonian drugs on Hcy concentrations. We compared Hcy, B(12) and folate levels in 45 PD patients (15 treated with dopamine-agonists, 15 with L-dopa and 15 with L-dopa plus a catechol-O-methyltransferase-inhibitor (COMT-I) and in 15 controls. Analysis of data revealed that L-dopa administration significantly increases Hcy concentrations and that the addition of COMT-I effectively reduces the homocysteinemia.