A woman had bilateral ptosis and rightsided ophthalmoplegia as prominent signs of a midbrain infarct. The lesion involved the third and fourth cranial nerve nuclei on the right. The lesion extended to the left side only at the caudal extent of the infarct, in the dorsal tegmentum of the mesencephalon. This midline lesion may be responsible for the bilateral ptosis, since this finding is consistent with current models of oculomotor organization in monkeys. To our knowledge, this is the first time such correlation has been demonstrated in man.