We compared the results of 123I-iodobenzamide (123I-IBZM) single photon emission computed tomography (SPECT) and 11C-raclopride positron emission tomography (PET) in 22 patients with parkinsonism. Nineteen patients were clinically diagnosed as having Parkinson's disease, two patients presented with atypical parkinsonism (i.e. clinical signs of Parkinson's disease and a subsequent poor response to dopamimetic drugs) and one patient was diagnosed as having Wilson's disease. All patients were drug naive. The SPECT data were semiquantitatively evaluated by calculating the ratios of striatal (basal ganglia, BG) IBZM binding to IBZM binding in various reference areas, i.e. the frontal cortex (BG/FC), the occipital cortex (BG/OC) and the cerebellum (BG/CE). In PET studies similar regions of interest were derived and ratios of striatal to cerebellar raclopride activity were determined. 123I-IBZM SPECT results significantly correlated to specific 11C-raclopride binding. This correlation was not significantly different when the frontal cortex (P = 0.05, r = 0.42), occipital cortex (P < 0.05), r = 0.44) or cerebellum (P 0.05, r = 0.45) were used as reference regions for non-specific IBZM binding. In comparison to PET ratios the SPECT BG/CE showed a higher variance (S.E.M. = 0.1) than BG/FC (S.E.M. = 0.05) and BG/OC (S.E.M. = 0.06) ratios. Thus, for the calculation of specific striatal IBZM binding one would prefer the frontal or occipital cortex as reference. However, when only those 19 patients with a clinical diagnosis of Parkinson's disease and increased specific 11C-raclopride binding were considered, no significant correlation was obtained. Qualitative changes of dopamine D2 receptor binding, such as asymmetry, were equally recognized.(ABSTRACT TRUNCATED AT 250 WORDS)