OBJECTIVE:Tremor is the most common movement disorder which is most often either essential or caused by Parkinson's disease. The differentiation of these disorders at the initial stage may be difficult. Objective assessment of the efficacy of tremor medications is only possible by instrumental measurements. The aim of this study was to determine whether the computer assisted tremor analysis system CATSYS 2000 can help in the differentiation of parkinsonian from essential tremor. METHODS:The rhythmicity and maximal frequency of fast alternating hand and finger movements, simple reaction time and postural instability were recorded in healthy controls (n = 18), patients with Parkinson's disease (n = 39) and essential tremor (n = 37). Data were digitally converted and statistically analyzed. RESULTS:Tremor intensity, median frequency and frequency distribution showed characteristic differences in the three groups. Performance in fast alternating movements of hands and fingers were significantly worse in both tremor groups compared to the healthy controls. CONCLUSIONS:The data also indicated that quantitative measurements of tremor parameters must be performed on both sides, because the presence of significant side differences support the diagnosis of Parkinson's disease. The method presented can be used to objectively analyze tremor and performance in rhythmic movements. The results show that it helps to differentiate parkinsonian from essential tremor as well as to predict disease course and the effectiveness of therapy. Multivariate statistical analysis of tremor and movement performance also provides an opportunity to study the pathogenesis of human tremor.