The clinical management of Parkinson's disease has been revolutionized by the introduction of levodopa therapy. It has significantly reduced disability and has extended life expectancies of patients with Parkinson's disease. However, motor response fluctuations frequently appear in patients after long-term treatment with levodopa. In this study, we investigated the effect of protein-restricted diet on fluctuations in eight patients with Parkinson's disease who had been receiving long-term levodopa treatment (mean 12.5 years). Two weeks of protein-restricted daytime diet (7.5 g total at breakfast and lunch) was followed by 12.5 g total at breakfast and lunch. At night, high-protein diet (40-50 g at dinner) was offered to the patients in order to maintain total daily protein intake at Japanese standard level. The medication schedule of levodopa and other antiparkinsonian drugs was not changed within 2 weeks after the study was began. Fluctuations were reduced in 7 of the 8 patients. But in only one patient (case 6), dyskinesia and general condition got worse and stopped this therapy. Body weight, serum protein and albumin levels did not change significantly for at least three month after the study was begun in every 6 patients who were examined. Homovanillic acid level of cerebrospinal fluid reduced in every 4 patients who were examined. We concluded that protein-restricted diet during the daytime offers a fascinating technique for the control of motor response fluctuations in patients with Parkinson's disease undergoing long-term levodopa treatment. But this therapy must be indicated carefully. Mechanism of this therapy may has something to do with improvement of dopamine metabolism in the brain.