A right-handed, 77-year-old man, complaining of dizziness, was diagnosed as agenesis of the corpus callosum (ACC) by MRI, which demonstrated a complete defect of the corpus callosum (CC) with symmetrical enlargement of the posterior horns of the bilateral ventricles (colpocephy) and focal hypo-plasticity of the frontal lobes. Neurological examination revealed only mild impairment of mental function and poor transfer of tactile form-board learning from the right hand to the left hand. As to the cerebral circulation detected by 123I-IMP SPECT, isotope distribution corresponded to the MRI brain images, though the defects were clearly seen on the medial surfaces of the bilateral frontal lobes, so interhemispheric fissure seemed to be opened widely. IV-DSA revealed hypo-vascularity in the anterior territory of the anterior cerebral arteries. These findings suggested that this localized defect was due to hypo-vascularity and focal hypo-plasticity of the frontal lobes. On the other hand, cine-mode MRI findings that no signal-void phenomenon was found between the third ventricle and the longitudinal fissure indicated no shunt in cerebrospinal fluid.