Differentiation between Parkinson's disease (PD) and other neurodegenerative disorders with parkinsonian features, such as multiple system atrophy (MSA), progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD), is difficult on clinical grounds. We studied the pattern of dopaminergic degeneration in 18 patients with probable MSA, 8 patients with PSP, 4 patients with CBD, 48 patients with PD and a similar degree of disability, and 14 control subjects performing single photon emission computed tomography (SPECT) 20 hours after injection of [123I]beta-CIT. Overall striatal binding was significantly reduced in MSA (-51% of normal mean), PSP (-60%), CBD (-35%), and PD (-58%), without overlap with control values. Asymmetry of striatal beta-CIT binding was significantly increased in patients with CBD and PD, as compared with control subjects. Although asymmetry seemed to be less pronounced in MSA and PSP than in PD, this was not statistically significant. Putamen-caudate nucleus ratios in patients with PD, MSA, and PSP, but not with CBD, were significantly reduced, as compared with control subjects. In conclusion, [123I]beta-CIT SPECT reliably enables the visualization of the presynaptic dopaminergic lesion in patients with MSA, PSP, and CBD. In most patients, however, it does not seem to be possible to differentiate these disorders from PD with this method.