The largest group of neurodegenerative disorders are extrapyramidal diseases, especially parkinsonism. The development of the cocaine derivative [123I] beta-CIT and single photon emission tomography (SPET) may help in the diagnosis of these patients. The aim of this study was to demonstrate the diagnostic value of this method and its relationship with clinical data. Ninety-eight individuals were investigated: 11 healthy volunteers, 58 patients with idiopathic Parkinson's disease (IPD) and 29 patients with symptomatic parkinsonism (SPD). All patients with parkinsonism were staged according to the clinical classification of Hoehn and Yahr. [123I] beta-CIT was injected intravenously and a triple-headed camera was used to obtain images 20 h later. The images were evaluated visually and semi-quantitatively to obtain comparable values (ratio: specific to non-displaceable binding). The ratios differed significantly between controls and IPD patients. A significant correlation also existed between the ratios and clinical stages. In 11 hemiparkinsonian patients, a significantly diminished ratio was demonstrated not only contralateral to the affected side, but also in the clinically silent striatum. A clinical threshold at a reduction of 34% [123I] beta-CIT binding was calculated in this group. The ratios of all SPD patients in our study did not differ significantly from those of the healthy volunteers. According to the clinical degree of symptoms, the more severe subgroup showed a diminished mean ratio of 22% and therefore could not be clearly differentiated from mild IPD. In contrast, ratios were significantly different when comparing groups of the same clinical severity. We conclude that this method is not only a powerful diagnostic tool in IPD patients, but it is also possible to differentiate between IPD and SPD patients, if clinical aspects are also included.