To clarify the regions most contributory to the manifestation of left unilateral ideomotor apraxia (LUIMA), clinical studies were performed in 12 consecutive patients with cerebral infarction confined to the unilateral anterior cerebral artery (ACA) territory. LUIMA was observed in 3 patients, all of whom had left ACA infarction involving the anterior four-fifths of the corpus callosum. LUIMA was absent in 5 other patients with left ACA infarction invading only the anterior half of the corpus callosum. The posterior trunk of the corpus callosum may thus be most crucial for the manifestation of LUIMA.