A number of neurodegenerative diseases have been evaluated with 123I-iodobenzamide (123I-IBZM) dopamine receptor scintigraphy, including Parkinson's disease. Differential diagnosis is based on the semi-quantitative determination of striatal uptake in the basal ganglia. Seven procedures for calculating basal ganglia uptake were compared and checked statistically in (1) 28 previously untreated de novo parkinsonian patients before and (2) 14 patients after (mean of 9 months) commencement of anti-Parkinson medication. Of the 21 hemi-parkinsonian patients, 16 demonstrated increased uptake contralaterally (mean right-to-left difference = 8%, sensitivity = 76%) using the most robust uptake procedure. The difference in uptake between the affected and contralateral sides (mean = 6%) was significant (P = 0.02). The mean (+/- S.D.) basal ganglia/frontal cortex (BG/FC) ratio was 1.55 +/- 0.14 (attenuation-corrected). Attenuation correction did not affect the relative ratio of basal ganglia uptake (P = 0.01). The anti-Parkinson medication did not result in any significant changes in the BG/FC ratio at follow-up, but responders could be differentiated from non-responders based on initial uptake (mean BG/FC ratio of 1.58 and 1.39 respectively). We conclude that 123I-IBZM can be used routinely to identify which Parkinson patients will benefit from dopaminergic medication.