Amyotrophic lateral sclerosis and Parkinsonism-dementia complex of the Kii peninsula (Kii ALS/PDC) is an endemic and a tauopathy, which shows clinical symptoms of amyotrophy, parkinsonism, and dementia. The objective of this study was to report the role of oxidative stress on Kii ALS/PDC using biochemical analysis. Urinary 8-hydroxydeoxyguanosine (8-OHdG)/creatinine ratio was analyzed in 11 patients with Kii ALS/PDC and 8 normal controls. The mean level of urinary 8-OHdG/creatinine ratio of the patients with Kii ALS/PDC was significantly higher than that of control subjects. Oxidative stress may be implicated in pathogenesis of Kii ALS/PDC.