Background: Large-scale methylmercury poisonings have occurred in Japan (Minamata and Niigata) and in Iraq. The current WHO threshold for adult exposure (hair level: 50 mu g/g) was based on evidence from Niigata, which included only acute and severe cases. That study leaves open the possibility of more subtle effects at lower exposure levels.Methods: The Shiranui sea had been contaminated in the 1950s by the discharge of methylmercury from a factory near Minamata.In 1960, the hair mercury content of 1694 residents living on the coastline of the Shiranui sea was measured by researchers from the Kumamoto Prefecture Institute for Health Research. Independently, in 1971, a population-based study to examine neurologic signs was conducted in the Minamata and Goshonoura areas, on the coastline of the Shiranui Sea, and the Ariake area (reference), by researchers at Kumamoto University. We identified 120 residents from exposed areas who were included in both datasets, plus 730 residents of Ariake (an unexposed area) who were also examined for neurologic signs.Results: Hair mercury levels were associated with perioral sensory loss in a dose-response relationship. The adjusted prevalence odds ratios and 95% confidence intervals for perioral sensory loss, compared with the lowest exposure category (0-10 mu g/g), were 4.5 (0.5-44), 9.1 (1.0-83), and 10 (0.9-110), for the dose categories >10 to 20, >20 to 50, and >50 mu g/g, respectively. The prevalence of all neurologic signs was higher in the exposure area than in Ariake.Conclusions: An increased prevalence of neurologic signs, especially perioral sensory loss, was found among residents with hair mercury content below 50 mu g/g.