The effects of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), a neurotoxin that produces the symptoms of Parkinson's disease, can be fully prevented in experimental animals by inhibiting monoamine oxidase B. On the basis of this observation, a double-blind, placebo-controlled study in patients with early Parkinson's disease was initiated to determine whether deprenyl (a selective monoamine oxidase B inhibitor) would delay the need for L-dopa therapy by slowing the progression of the disease. Fifty-four patients were randomly assigned to deprenyl (10 mg/day) or placebo treatment groups and followed until L-dopa therapy was indicated or until the patient had been in the study for 3 years. Analysis of Kaplan-Meier survival curves for each group showed that deprenyl delayed the need for L-dopa therapy; the average time until L-dopa was needed was 312.1 days for patients in the placebo group and 548.9 days for patients in the deprenyl group. Disease progression, as monitored by five different assessment scales, was slowed (by 40 to 83% per year) in the deprenyl group compared to placebo. Therefore, early deprenyl therapy delays the requirement for antiparkinsonian medication, possibly by slowing progression of the disease.