It has been reported that the antiparkinsonian efficacy of levodopa is reduced after long-term administration However, main parkinsonian motor symptoms, since they are mainly caused by the deficiency of nigrostriatal dopamine, should be corrected if sufficient dopamine is supplied exogenously. Therefore, the functional decline in patients with long-term levodopa therapy may result either from side effects such as response fluctuation and abnormal involuntary movements or from progression of doPa -unresponsive parkinsonian symptoms, instead of reduction in levodopa efficacy itself. To adress this question, we measured residual motor disability during and at 6 hours after continuous intravenous levodopa infusion with optimal dose for at least 16 hours in 54 patients with idiopathic Parkinson's disease. While the basal motor disability is increased according to the advance of symptom duration as well as Hoehn and Yahr stage, the residual motor disability after levodpa supplement is not changed. The duration of levodopa therapy until development of motor fluctuation is significantly shorter in good responder (residual motor disability<2. 0) than in poor r-ponder(residual motor disability>2.0), and positively correlated to the residual motor disability. These findings suggest; first, the functional decline observed in parkinsonian patients with chronic levodopa therapy mainly results from motor fluctuation and/or progression of dopa-unresponsive symptoms, not from decline of levodopa efficacy itself on cardinal motor symptoms; second, the parkinsonian patients with good levodopa response may develop motor fluctuation earlier than those with poor response.