Asperger's syndrome has many clinical features in common with acquired right-hemisphere dysfunction and has been postulated to result from a developmental abnormality of the right hemisphere. However, right-hemisphere abnormality has not previously been documented on neuroanatomic or functional imaging in patients with Asperger's syndrome. We report three patients with Asperger's syndrome found to have abnormal right-hemisphere function on single photon emission computed tomographic (SPECT) imaging. The subjects were two males and one female, ranging from 12 to 16 years of age. All were diagnosed on the basis of the presence of the complete constellation of clinical features previously outlined. All patients were investigated with computed tomographic (CT) scanning, magnetic resonance imaging (MRI), and SPECT scanning. In one subject, CT and MRI revealed enlargement of the right lateral ventricle, reflecting a mild degree of right hemispheric atrophy. CT and MRI studies on the other two subjects were normal. SPECT scanning demonstrated right hemispheric abnormalities in each subject: right temporal hypoperfusion with a central area of increased perfusion along with frontal polar hyperperfusion in one; diffusely decreased right hemispheric uptake in the second; and decreased frontal and occipital uptake in the third. Cerebellar abnormalities were also present: a smaller right hemisphere with increased uptake in the first; decreased uptake in the vermis and right hemisphere in the second; and decreased vermal uptake in the third. These findings support the hypothesis that the neurobiologic basis of Asperger's syndrome is a developmental abnormality of the right cerebral hemisphere.