The natural occurrence of antiparkinsonian drugs in plants--anticholinergics in Datura stramonium, levodopa in Mucuna pruriens and Vicia faba, dopamine agonist activity in Claviceps purpura, and MAO inhibitor activity in Banisteria caapi-are known. Our study examined the efficacy and tolerability of HP-200, derived from Mucuna prurient, in patients with Parkinson's disease. Sixty patients with Parkinson's disease (46 male and 14 female) with a mean (+/- SD) age of 59 +/- 9 years were treated in an open study for 12 weeks. Of these, 26 patients were taking synthetic levodopa/carbidopa formulations before treatment with HP-200, and the remaining 34 were levodopa naive. HP-200, a powder (supplied as a 7.5 g sachet), was mixed with water and given orally. The Unified Parkinson's Disease Rating Scale (UPDRS) was used at baseline and periodically during the 12-week evaluation. Statistically significant reductions in Hoehn and Yahr stage and UPDRS scores were seen from baseline to the end of the 12-week treatment (p < 0.0001, t-test). The group mean (+/- SD) dose for optimal control of symptoms was 6 +/- 3 sachets. Adverse effects were mild and were mainly gastrointestinal in nature. No adverse effects were seen in clinical laboratory reports. HP-200, developed from an alternative medicine source, Ayurveda, was found to be an effective treatment for patients with Parkinson's disease.