Unilateral hyperhidrosis occurred contralateral to acute cerebral infarctions in six patients. Two patients had localized infarctions of opercular cortex, while the other four had large strokes involving both superficial cortical and deep subcortical structures. The unilateral hyperhidrosis typically involved the face and arm and was transient, lasting 1 to 3 days. No other autonomic dysfunction occurred. One patient died, and the others had significant residual neurologic disability, thus indicating poor prognosis when this sign is present. Disruption of a pathway of cortical origin, inhibitory to contralateral sweating, is a proposed mechanism for the hyperhidrosis seen contralateral to acute cerebral infarction in these patients.