Phantom measurements were performed with a conventional single-head single-photon emission tomography (SPET) camera in order to validate the relevance of the basal ganglia/frontal cortex iodine-123 iodobenzamide (IBZM) uptake ratios measured in patients. Inside a cylindrical phantom (diameter 22 cm), two cylinders with a diameter of 3.3 cm were inserted. The activity concentrations of the cylinders ranged from 6.0 to 22.6 kBq/ml and the cylinder/background activity ratios varied from 1.4 to 3.8. From reconstructed SPET images the cylinder/background activity ratios were calculated using three different regions of interest (ROIs). A linear relationship between the measured activity ratio and the true activity ratio was obtained. In patient studies, basal ganglia/frontal cortex IBZM uptake ratios determined from the reconstructed slices using attenuation correction prior to reconstruction were 1.30 +/- 0.03 in idiopathic Parkinson's disease (n = 9), 1.33 +/- 0.09 in infantile and juvenile neuronal ceroid lipofuscinosis (n = 7) and 1.34 +/- 0.05 in narcolepsy (n = 8). Patients with Huntington's disease had significantly lower ratios (1.09 +/- 0.04, n = 5). The corrected basal ganglia/frontal cortex ratios, determined using linear regression, were about 80% higher. The use of dural-window scatter correction increased the measured ratios by about 10%. Although comprehensive correction methods can further improve the resolution in SPET images, the resolution of the SPET system used by us (1.5-2 cm) will determine what is achievable in basal ganglia D2 receptor imaging.