No significant increase of the maximum concentration (Cmax) of levodopa after addition of catechol-O-methyltransferase inhibitor tolcapone occurred in previous pharmacokinetic studies predominantly on healthy volunteers. We compared pharmacokinetics of levodopa in plasma before and after addition of tolcapone in 13 treated parkinsonian subjects under standardized conditions. We found a significant increase of Cmax of levodopa after the addition of tolcapone. This may represent one cause for the occurrence of dyskinesia previously early in the course of treatment with tolcapone.