Iodine-123-IBF is a dopaminergic antagonist suitable for SPECT imaging of D2 receptors. Initial animal studies demonstrated that its affinity for D2 receptors is approximately four times that of the commonly used SPECT D2 ligand [123I]IBZM. In this study we investigated whether this higher affinity would lead to an improved accuracy in differentiating between various extrapyramidal diseases. METHODS SPECT imaging was performed in 17 patients with idiopathic Parkinson's syndrome (IPS); 4 patients with progressive supranuclear palsy (PSP), 2 patients with multiple system atrophy (MSA) and 7 age-matched control subjects. SPECT imaging was performed 5, 60, 120 and 180 min following intravenous bolus injection of 150-250 MBq of [123I]IBF. The ratio of ligand uptake in the basal ganglia and frontal cortex was determined as a measure of receptor status. RESULTS In PSP and MSA patients, the basal ganglia-to-frontal cortex ratio reached a plateau at 2 hr; in the control subjects and the IPS patients the ratio was steadily increasing. At 3 hr the basal ganglia-to-frontal cortex ratio was 2.66 +/- 0.29 (control subjects), 3.01 +/- 0.41 (IPS), 2.09 +/- 0.22 (PSP) and 2.10 (MSA). In the IPS patients with predominantly one-sided symptoms, the striatum contralateral to symptoms showed a tendency towards relatively increased ligand uptake. Despite the higher affinity of IBF for the D2 receptor compared to IBZM, the separation of individual PSP and MSA patients from the control subjects was not as clear cut as reported for IBZM due to a relatively high variation in the control subjects. We hypothesize that the latter is due to imaging in nonequilibrium conditions. CONCLUSIONS The data suggest that IBF-SPECT can help in discriminating extrapyramidal disease. The accuracy might be improved by an administration protocol that allows imaging in "true equilibrium" conditions, such as a bolus injection followed by a constant infusion.