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Abstract Atypical parkinsonian syndromes (APS) such as multiple system atrophy, progressive supranuclear palsy, and corticobasal degeneration are characterized by poor response to antiparkinsonian medication and rapid clinical deterioration. We used SPECT and [ 123 I]β‐CIT as a label of dopamine transporters to study the progression of presynaptic dopaminergic degeneration in Parkinson's disease (PD) and APS. Twenty‐four PD patients with short disease duration (2.4 ± 1.5 years), 12 PD patients with long disease duration (9.2 ± 2.6 years), 10 patients with APS (disease duration 2.1 ± 1.5 years), and nine patients with essential tremor (ET) underwent sequential [ 123 I]β‐CIT SPECT imaging with an interval of 25.5 ± 10.3 (13–63) months. The age‐related decline of striatal β‐CIT binding was studied cross‐sectionally in 30 healthy subjects. The ratio of striatum/cerebellum −1 at 20 hours after tracer injection, reflecting specific‐to‐nondisplaceable binding, was used as the primary SPECT outcome measure. At scan 1, striatal β‐CIT binding was reduced in PD patients with short disease duration (−42% compared with age‐corrected normal values) and long disease duration (−51%), and APS (−36%), but normal in ET. During the observation period striatal β‐CIT binding significantly declined in patients with APS (14.9% per year) and short duration PD (7.1% per year), whereas PD patients with long disease duration and patients with ET showed no significant change of striatal β‐CIT binding between scans 1 and 2. The relative annual reduction from age‐corrected normal values at the time of scan 1 was significantly higher in patients with APS than in PD patients with short disease duration (9.6 vs. 4.3%, P = 0.004). These results demonstrate a rapid decline of striatal β‐CIT binding in patients with atypical parkinsonian syndromes, exceeding the reduction in PD. The dopaminergic degeneration in PD appears to slow down during the course of the disease. SPECT with [ 123 I]β‐CIT is a sensitive marker of disease progression in parkinsonian disorders. © 2001 Movement Disorder Society.