Single photon emission computed tomography (SPECT) with 123I-iodobenzamide (123I-IBZM) was used in a prospective study to investigate 83 patients with parkinsonism (Hoehn and Yahr stages I to III) who had not been previously treated with dopamimetic drugs. All patients had clinical signs that were compatible with Parkinson's disease. An additional 13 patients had clinical signs of another basal ganglia disorder, such as progressive supranuclear palsy or multisystem atrophy. 123I-IBZM-SPECT results were compared with clinical responses to subcutaneous injections of the D1/D2-receptor agonist apomorphine (83 patients) and to long-term oral dopamimetic therapy (62 patients). Results from 123I-IBZM-SPECT predicted a positive or negative response to apomorphine in 69 of 76 patients (apomorphine responses were equivocal in 7 patients) and a response to dopamimetic therapy in 54 of 62 patients. All patients with a clinical diagnosis of progressive supranuclear palsy or multisystem atrophy had reduced 123I-IBZM binding. In six of these patients, the response to apomorphine was negative, and none clearly benefited from long-term oral levodopa therapy. Imaging of dopamine D2 receptors with 123I-IBZM-SPECT appears to distinguish between patients with de novo parkinsonism that is levodopa-responsive (probably Parkinson's disease of Lewy body type) and that which does not respond to levodopa therapy.