The objective of this paper is to evaluate the accuracy of conventional diagnostic criteria for Parkinson's disease and give an overview of alternative causes to parkinsonian syndromes. We also propose a new approach to the clinical diagnosis of Parkinson's disease, which may improve the diagnostic accuracy. The available information on autopsy findings in patients clinically diagnosed as Parkinson's disease shows that 20 to 30% of these patients do not have the typical neuropathological features with Lewy bodies and cell loss in the substantia nigra. The use of selected additional clinical criteria improves the diagnostic accuracy, however, at the cost of rejecting a rather large group of patients with idiopathic disease verified by autopsy. Based on this fact and a review of the literature on parkinsonian syndromes that may be confused with idiopathic Parkinson's disease, we propose criteria for diagnostic subgroups of the disease classified at different levels of confidence. The suggested diagnostic subgroups are clinical definite, probable and possible Parkinson's disease with a decreasing level of specificity and an increasing level of sensitivity in the different patient categories. The clinical features given special importance in this classification includes presence of resting tremor, asymmetrical disease, response to dopamine agonism and presence of atypical clinical features like dementia and clinical autonomic failure at onset and pyramidal or cerebellar signs at examination. In addition, a history indicating possible etiology for another parkinsonian syndrome will exclude the diagnosis.