Thirteen patients suffering from motor neuron disease with dementia were studied to analyze the clinicopathological spectrum. The diagnosis of the disease was made on the basis of a clinical history of progressive dementia and motor neuron involvement. The mean age at onset of 11 sporadic cases was 54.9 years (range, 43 to 69 years), with a mean duration of disease of 25 months (range, 11 to 47 months). The initial symptoms were dementia in 7 cases, motor neuron involvement in 2 cases, and both dementia and motor neuron involvement in 2 cases. The clinical picture of motor neuron disturbance in sporadic cases represented bulbar-type of amyotrophic lateral sclerosis (ALS). Bulbar palsy was the initial symptom in 7 sporadic cases and all 11 patients developed bulbar palsy with advancing course of illness. Muscular wasting and fasciculation were more predominant in the upper limbs, shoulder girdle and anterior chest. Fasciculation was more extensively and frequently observed in those portions than that of classical ALS. In contrast, muscle strength in the lower limbs was well preserved so that all patients could walk even when respiratory failure developed. Hyperreflexia including jaw jerk was found in all cases and positive Babinski sign in 7 cases. Parkinsonism appeared in the initial stage in one sporadic case and in two familial cases. The type of dementia with uninhibited behavior and personality change closely mimicked that of Pick's disease. The degree of dementia was mild or moderate in 8 cases and severe in 3 cases. Language disorder was characterized by progressive reduction of speech output, leading finally to mutism in 5 cases. Perseveration was observed in 10 cases. Visuospatial disorder was absent even in the advanced stage. Mild memory disturbance was noted in the early stage in 10 cases. Pathological examination was performed in 7 cases including one familial case, revealing frontal atrophy in 3 cases, frontotemporal atrophy in 2 cases and temporal atrophy in 2 cases. On microscopic examination there were mild neuronal loss, gliosis, mild spongy state of the cortical superficial layers and fibrous gliosis in the frontotemporal white matter. The scattered senile plaques in one case did not justify a diagnosis of Alzheimer's type dementia. Neither circumscribed atrophy nor Pick body was found in any case. The nucleus basalis of Meynert showed no neuronal loss. The substantia nigra showed a mild to severe loss of nerve cells without Lewy bodies in all cases.(ABSTRACT TRUNCATED AT 400 WORDS)