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To evaluate the diagnostic value of putaminal abnormality on 3-T magnetic resonance imaging (MRI) for differentiating early parkinsonism-predominant multiple system atrophy (MSA-p) from Parkinson disease (PD) based on long-term clinical follow-up data. Totals of 23 clinical MSA-p (6 possible and 17 suspicious) and 50 PD patients were included. Subjects submitted to 3-T MRI at baseline and were followed up to substantiate the initial diagnosis. MRI findings were compared between MSA-p and PD patients based on the final diagnosis. Putaminal abnormalities were recorded as presence of atrophy, signal hypointensity, and abnormal disruption of the hyperintense lateral rim of the putamen. The sum scores for putaminal abnormality were calculated from the presence of each item. During the follow-up over 3 years, the diagnosis of MSA-p was supported in 17 patients (14 probable and 3 possible) and the diagnosis of PD was stable in all 50 patients. Putaminal abnormalities were more frequent in MSA-p (n = 17) than in PD (n = 50). A sum score of >1 on 3-T MRI had sensitivity, specificity, and positive- and negative-predictive values of 70.6, 93, 77.4, and 90.3%, respectively, for differentiating MSA-p from PD. Fourteen of the initial 23 clinical MSA-p patients had a sum score of >1, and in all but two, the diagnosis became supported during the follow-up, whereas the diagnosis of five of the nine patients with a sum score of ≤1 remained uncertain. Putaminal abnormality on 3-T MRI can be a specific diagnostic marker for early stage MSA-p.