We report here the severe involvement of the ambient gyrus in a case of argyrophilic grain (AG) dementia (AGD). The patient was a 78-year-old man who was first presented with prosopagnosia (agnosia of the face) at age 68, which was followed by progressive mental decline and the patient's death in a state of tetraplegia. The postmortem study showed severe atrophy of the medial temporal lobe with anterior gradient, most prominent in the ambient gyrus. Histologically, numerous AGs, pretangles and coiled bodies were detected by Gallyas-Braak (G-B) silver staining and also by immunostaining with various anti-tau antibodies in the affected area. Tau-immunoreactive ballooned neurons were also present. Neuronal loss and gliosis with laminar sponginess were evident in the ambient gyrus. Diffuse plaques were seen in the neocortex and frequently associated with clusters of AGs, which were morphologically distinct from neuritic plaques. Neurofibrillary tangles were localized in the entorhinal area. Vascular lesions were very scanty. Thus, this case fulfilled the morphological criteria of AGD. It is still unclear whether AG itself causes neuronal degeneration leading to dementia. The present case may reflect the importance of the ambient gyrus in the center of neuronal degeneration in AGD.