On a nearly zero protein diet, 11 patients with Parkinson's disease with the "on-off" effect demonstrated great sensitivity to levodopa (L-dopa)-carbidopa and reduced fluctuations. Eight patients required a 41% reduction in total L-dopa dosage and discontinuation of all adjuvant therapy to reduce the preponderance of chorea. On a high-protein diet, all patients were immobilized by bradykinesia for most of the day. A low-protein dietary regimen during the daytime offers an important technique for the control of fluctuations in patients with Parkinson's disease who are receiving L-dopa-carbidopa.