The first paper on the beneficial effect of selegiline in the treatment of Parkinson's disease was published 17 years ago. In the first ten years several clinical studies were conducted world-wide to prove its efficacy as an adjuvant to the basic 1-dopa therapy. Although the design, duration and number of treated patients were different, the overall results were mainly uniform. The patients treated with 1-dopa selegiline combination showed marked improvement in disability, increasing duration of 1-dopa effect and marked lessening of dose-related fluctuations were seen, compared to those treated with 1-dopa alone. It became also generally evident that selegiline allows a 10-30% decrease in 1-dopa dose. The investigations in which selegiline has been used as monotherapy have been triggered by some purely clinical experience and by the discovery of pathobiochemical mechanisms of MPTP toxicity which causes fairly similar clinical picture to Parkinson's disease and can be successfully prevented by selegiline. The newly diagnosed parkinsonian patients treated with selegiline alone showed marked improvement in their clinical state. Beside the actual clinical effect on disability, selegiline slowed down the progression of Parkinson's disease. The latter fact has been proven in the largest clinical trial ever done with selegiline (DATATOP). Selegiline monotherapy can delay the need for 1-dopa substitution by around one year or longer. Based on the continuously growing clinical experiences it became widely accepted that selegiline is the drug of choice as initial treatment for newly diagnosed parkinsonian patients.(ABSTRACT TRUNCATED AT 250 WORDS)