Clinicopathological data of a woman with a 3-year course of concurrent amyotrophic lateral sclerosis and dementia are presented. Dementia had occurred at time of onset of motor disturbances and presented as typical frontal lobe dementia. Pathology confirmed motor neuron disease of amyotrophic lateral sclerosis and frontal lobe atrophy. Multiple senile plaques were distributed cortically and in the hippocampus, where diffuse spread of neurofibrillary tangles was seen. Hence, this Alzheimer's dementia in a patient with sporadic amyotrophic lateral raises the question of a possible association between the two conditions.