Previous studies have utilized single-photon emission computed tomography (SPECT) to demonstrate decreased [123I]beta-CIT striatal uptake in idiopathic Parkinson disease (PD) patients. The present study extends this work by examining SPECT outcome measures in a larger group of PD patients with varying disease severity. Twenty-eight L-dopa-responsive PD patients (Hoehn-Yahr stages 1-4) and 27 healthy controls had SPECT scans at 18 to 24 hours after injection of [123I]beta-CIT. Specific to nondisplaceable striatal uptake ratios (designated V3") were correlated with Hoehn-Yahr stage and Unified Parkinson's Disease Rating Scale (UPDRS) subscores. Linear discriminant function analyses utilizing striatal uptakes, putamen-to-caudate ratios, and ipsilateral-contralateral asymmetry indices were performed. Decreased striatal tracer uptake (V3") was correlated with total UPDRS score for both contralateral and ipsilateral striatum. Putamen uptake was relatively more reduced than caudate with mean putamen:caudate ratios of 0.50 +/- 0.17 and 0.82 +/- 0.09 for PD patients and controls, respectively. Ipsilateral:contralateral asymmetry was significantly greater in PD patients than controls. Discriminant function analysis utilizing V3" for ipsilateral and contralateral caudate and putamen correctly classified all 55 cases. These data demonstrate marked differences in [123I]beta-CIT SPECT measures in healthy controls and PD patients. The significant correlation of SPECT measures with motor severity suggests [123I]beta-CIT may be a useful marker of disease severity in PD.