The effectiveness and tolerability of deprenyl as an adjunct in the therapy of parkinsonism were studied in a double-blind trial comprising 30 de novo patients. Quantification was based on the Columbia University Rating Scale, the SCHOPPE Motor Performance Series, as well as Zung's Scale. The results were evaluated using a response score. Two thirds of the cases that could be evaluated showed a statistically significant improvement while on adjuvant deprenyl therapy. One third of the patients showed no improvement or worsened. Using the Motor Performance Series, the improvements are most clearly shown in the replugging test, which is the best measure of the patient's ability to perform complex movements. This means that deprenyl is particularly useful in improving the akinesia-rigor syndrome. This study suggests that early combination of levodopa with deprenyl is superior to treatment with levodopa alone.