Asymmetric onset of parkinsonism appears not uncommon in SND. This study quantitatively investigated anatomical asymmetry of the putamen using MRI in relation to symptomatic laterality. Our study comprises six patients with SND, two men and four women, ranging in age from 50 to 77. Duration of illness ranged from 1 to 4 years. The Yahr stage was IV in 3 patients and I, III, or V in the others. The diagnosis of SND was made on medical history, neurological examination and MRI. All patients had asymmetric onset of parkinsonism. They subsequently had variable cerebellar ataxia and/or autonomic failure. All of them responded poorly to L-dopa therapy. MRI revealed the pontine and cerebellar atrophy and abnormalities of the putamen in most of the patients. MRI was performed using a 1.5 Tesla in 5 patients and 0.5 Tesla in one. Inversion Recovery (IR) image were utilized as T1-weighted images and proved optimal in showing anatomic detail. Spin-echo sequences were used to provide T2-weighted images. For measurement of the putaminal area and length in SND patients and 12 normal controls, we used two horizontal slices. One is passing through dorsal portion of the putamen, and the other through the ventral putamen. All the patients had the onset of hemi-parkinsonism and the symptoms progressed dominantly on the side of onset. With T1-weighted IR sequences, putaminal atrophy was demonstrated in all patients, more intense on the contralateral side to more symptomatic limbs. The anatomical asymmetry and the symptomatic laterality were less pronounced in advanced patients.(ABSTRACT TRUNCATED AT 250 WORDS)