L-Dopa-induced dyskinesias constitute a challenge to the management of advanced Parkinson's disease. According to recent reports, treatment with the NMDA receptor antagonist amantadine may significantly diminish L-dopa-induced dyskinesias. In the present study, the effect of amantadine on L-dopa-induced dykinesias was assessed in a 5-week, double-blind crossover trial. Dyskinesia severity as assessed following oral L-dopa challenges and by self-scoring dyskinesia diaries were reduced approximately 50% after amantadine treatment compared with baseline or placebo phases. Similarly, dyskinesia assessments on the Unified Parkinson's Disease Rating Scale, part IV (items 32 and 33) also revealed significant improvement after treatment with amantadine. The magnitude of the L-dopa motor response to oral challenges was not different after amantadine or placebo treatment, and there was no significant reduction of daily off-time when patients received active treatment. These results confirm previous observations concerning the antidyskinetic potential of amantadine.