Degeneration of the perforant pathway, not extensively surveyed so far in amyotrophic lateral sclerosis (ALS) with dementia, was found in eight out of twelve autopsied patients with clinically detectable dementia. Because the severity of degeneration of the entorhinal cortex and that of spongiosis of the outer half of the molecular layer of the dentate gyrus were correlated in these eight patients, it is suspected that the degeneration of the perforant pathway may explain these concomitant lesions. This was further corroborated by occasional involvement of the parahippocampal white matter and subiculum, other components of this pathway. Moreover, six of them manifested clinically detectable memory deficits and three of them exhibited amnesia or episodic memory impairments similar to Alzheimer's disease (AD). Abnormal intensity restricted the dentate gyrus on brain magnetic resonance imaging in a severe case looks like the degeneration of the molecular layer. This involvement of the perforant pathway in ALS patients and its correlation to memory deficits should be taken in account for evaluation of dementia.