α-Synuclein pathology was examined in the brains and spinal cords of 10 patients with amyotrophic lateral sclerosis (ALS)/parkinsonism-dementia complex (PDC) in the Kii Peninsula, Japan. Various types of phosphorylated α-synuclein-positive structures including neuronal cytoplasmic inclusions, dystrophic neurites, and glial cytoplasmic inclusions were found in all ALS/PDC cases. There were phosphorylated α-synuclein-positive neurons in 8 cases (80%), and the amygdala was most severely affected. Phosphorylated α-synuclein was distributed mainly in the limbic system and brainstem; tau pathology was more prevalent than α-synuclein pathology in most affected areas. In the substantia nigra, periaqueductal gray, locus coeruleus, raphe nuclei, dorsal nucleus of the vagus nerve, hypoglossal nucleus or ventral horn, and intermediolateral nucleus of the spinal cord, α-synuclein pathology was more predominant than tau pathology in only 1 or 2 patients. Phosphorylated α-synuclein- positive structures were not found in the molecular layer of the cerebellum. Phosphorylated α-synuclein frequently colocalized with tau in neuron cell bodies, neurites, and glia. Immunoblots of sarkosyl-insoluble fractions extracted from the brain of 1 patient showed a triplet of α-synuclein-immunoreactive bands that were ubiquitinated. These results suggest that interaction between tau and α-synuclein be involved in the pathogenesis of Kii ALS/PDC.