Since an oral regimen of levodopa has been instituted for treatment of Parkinson's disease, its absorption and metabolism has been well demonstrated. However, its chemical characteristics of high solubility in acid solution and low solubility in water have not been well known. We paid attention to this characteristic and studied the relationship between its absorption and gastric acid secretion in 38 patients with Parkinson's disease who became refractory to therapy of levodopa. We measured the pH and amount of collected fasting gastric juice. Gastric acid secretion was decreased in 22 patients (58%). In ten of these 22 patients, 30 ml of lemon juice was prescribed in every administration of levodopa as a supplement to gastric acid for two weeks. Increases of L-dopa concentration after 60 min. and 180 min. were observed after lemon juice supplement therapy. Among the Parkinson symptoms, rigidity, akinesia, and small step gait were improved in every case except one patient who showed decrease of L-dopa concentration at 180 minutes. However, improvement of tremor was less remarkable. We consider this supplement therapy to gastric acid is one of the effective and useful methods in the management of Parkinson's disease.