A family with four affected members showed a stereotyped unilateral neurologic deficit. The disorder was characterized in each member by congenital left hemiparesis, with subsequent development of left hemihypoplasia and athetoid posturing of the left hand. Although the proband was initially considered a “cerebral palsy” victim, the autosomal dominant pattern and the stereotyped clinical features in all affected family members suggest a hereditary process. Hereditary factors may be an uncommon cause of static unilateral neurologic motor defects.