A multicenter trial was conducted at 9 Neurology Departments to evaluate the action of L-Deprenyl, a specific monoamine oxidase-B inhibitor, combined with L-Dopa in the treatment of Parkinson disease. In all, 76 patients were treated, 33 women and 43 men, on stable treatment with L-Dopa+ aromatic decarboxylase inhibitors (DI) for at least 6 months. After a 50% reduction of the L-Dopa dose, all received L-Deprenyl 5 mg twice daily for 35 day. The combined treatment resulted in a definite improvement in rigidity, bradykinesia and, most of all, tremor. Further, at the end of treatment fewer patients had depressive symptoms and the total daily number of hours of wellbeing and normal movement increased. 12 patients presented modest side effects, in no case serious enough to warrant suspension of treatment. The trial shows that with the L-Deprenyl + L-Dopa combination the dose of L-Dopa needed to control the disease can be drastically reduced.