Large-scale food poisoning caused by methylmercury was identified in Minamata, Japan, in the 1950s (Minamata disease). Although the diagnostic criteria for the disease remain current, few studies have been carried out to assess the diagnostic accuracy of the criteria. From a 1971 population-based investigation, data from 2 villages were selected: Minamata (high-exposure area; n = 779) and Ariake (low-exposure area; n = 755). The authors examined the prevalence of neurologic signs characteristic of methylmercury poisoning and the validity of the criteria. A substantial number of residents in the exposed area exhibited neurologic signs even after excluding officially certified patients. Using paresthesia of the extremities as the gold standard of diagnosis, the criteria had a sensitivity of 66%. The current diagnostic criteria as well as the official certification system substantially underestimate the incidence of Minamata disease.