We report a 72-year-old man with sporadic amyotrophic lateral sclerosis (ALS) who showed concomitant histopathology of Alzheimer's disease (AD) and incidental Lewy body disease. The patient presented at the age of 70 years with distal upper limb amyotrophy. Thereafter, gait disturbance and respiratory distress progressed. Neuropathological examination showed mild frontal lobe and anterior spinal root atrophy. There was moderate loss of upper and lower motor neurons, and Bunina bodies and skein-like inclusions were present in the spinal anterior horns and facial and hypoglossal nuclei, confirming the pathology of ALS. In addition, however, numerous amyloid plaques were observed throughout the entire cerebral neocortex, nucleus accumbens and amygdaloid body. Many neurofibrillary tangles were also evident in the medial temporal cortex. Moreover, the substantia nigra showed mild degeneration, and Lewy bodies were found in the substantia nigra, locus ceruleus, basal nucleus of Meynert and peripheral autonomic ganglia. Although neither parkinsonism nor dementia was noted during the clinical course, our final neuropathological diagnosis was sporadic ALS, AD and incidental Lewy body disease (or presymptomatic Parkinson's disease). Whether or not the coexistence of these three diseases in the same patient was merely coincidental is of considerable interest.