We report the first case of neuropathologically verified parkinsonism-dementia complex of the Kii peninsula, together with the patient's brother, who had amyotrophic lateral sclerosis. The propositus woman developed parkinsonism and dementia at 63 years of age and died at 70 without displaying clinical features of amyotrophic lateral sclerosis. The brain exhibited marked atrophy of the frontal and temporal lobes. Microscopically, there were many neurofibrillary tangles in the central nervous system, most markedly in the mesial temporal lobe and deep nuclei, as well as changes of amyotrophic lateral sclerosis but no senile plaques or Lewy bodies. Neurofibrillary tangles exhibited twisted tubule structures on electon microscopic examination, and an analysis of insoluble tau protein extracted from the fresh brain revealed a 60-, 64-, 68-kD triplet. The tau gene exhibited no mutations. Her brother developed progressive bulbar palsy-type amyotrophic lateral sclerosis at 45 years of age and died at 49 without presenting with dementia or parkinsonism. Neuropathological examination revealed not only pathologic features of amyotrophic lateral sclerosis but also a moderate number of neurofibrillary tangles in the temporal cortex and deep nuclei. The siblings were neuropathologically similar despite their different clinical manifestations. These findings suggest that amyotrophic lateral sclerosis and parkinsonism-dementia complex of this family may be phenotypic variants of a tauopathy caused by genetic abnormalities.