A patient with acute onset "classic" cerebellar ataxia of the right arm without clinically detectable deep sensory loss is reported, in relation to an acute posterior parietal infarct. Wild back and forth swaying of the arm, giving away, or worsening by suppression of vision were not seen. The lesion involved area 5, parts of area 7, the angular gyrus, the middle and posterior parieto-occipital gyri, and posterior parts of the superior and middle temporal gyri. The paracentral lobule, commonly thought to be responsible for parietal ataxia, was spared. Thus posterior parietal lesions can mimick cerebellar ataxia, possibly by severing specific projections to the ventrolateral thalamic nuclei. On the basis of previous studies in primates, the superior parietal gyrus may play a major part in the ataxia presented by this patient.