A morphological study using a modified Golgi impregnation technique was carried out on Betz cells in the motor area of eight patients with motor neuron disease (MND) and age-matched controls without abnormal neuropathological findings during routine examination. Betz cells were categorized as normal (type I), or those with moderate (type II) or marked (type III) degeneration of dendrites. Types II and III Betz cells tended to be dominant in the cases of MND, whereas type I Betz cells constituted the highest percentage in most of the control cases. The frequently observed morphological alterations of Betz cells in MND were accumulation of lipofuscin, degeneration or loss of dendrites, and reactive astrocytic gliosis around the Betz cell soma. It was suggested that Betz cell changes in MND are not only due to aging but also to evident pathological processes, since the above-mentioned findings were more extensive in the MND cases than in the control cases. Also, the socalled lower motor neuron disease belongs to one basic disease termed “motor neuron disease”, because the pathological changes of Betz cells were observed even in cases of spinal progressive muscular atrophy alone or associated with progressive bulbar palsy, which are devoid of pyramidal tract degeneration and signs of upper motor neuron lesion.