20 new cases of ataxic hemiparesis syndrome (AHS) are reported and the findings compared with those of published cases. AHS may be due to lesions either of the brainstem or of supratentorial structures, where motor fibers run together with the cerebro-cerebellar pathways. Specific syndromes related to the lesion site cannot be identified from the clinical signs. Ischemic infarct is the most frequent cause of the syndrome, but hemorrhagic, neoplastic and demyelinating lesions have also been reported.