This report conveys the results of an immunocytochemical and ultrastructural study of the motor cortices of six patients with clinically and pathologically-diagnosed lower motor neuron disease (LMND) such as progressive spinal muscular atrophy, progressive bulbar palsy, or both. These patients showed neither upper motor neuron signs nor upper motor neuron system involvement including the corticospinal tract in postmortem tissues after conventional stainings. Specimens from 12 age-matched normal individuals served as controls. All patients showed loss of brainstem motor neurons and anterior horn cells. Betz cells in LMND patients were significantly reduced in number as compared to controls (P<0.01). However, there was no significant difference in the density of phosphorylated neurofilament (PNF) (200 kDa)-positive Betz cells between LMND patients and controls. The pyramidal cells of layer III were immunostained for PNF in four of six LMND patients, but there was no significant difference in the density of PNF-positive pyramidal cells between LMND patients and controls. The number of astrocytes immunostained for glial fibrillary acidic protein increased in layer III and at the transition between white matter and motor cortex in three out of six patients and one of 12 controls. Ultrastructural examination revealed that the Betz cells of five of six LMND patients had Bunina bodies, Lewy body-like inclusions or skein-like inclusions, all of which are characteristic of amyotrophic lateral sclerosis (ALS). These findings suggest that most patients with clinically and pathologically-diagnosed LMND should be classified into the category of ALS.