On May 1, 1956, a local public health center in Minamata, Kumamoto Prefecture, reported the occurrence of a "rare disease of unknown cause" afflicting four people who showed symptoms of an unexplained brain disorder. This was the first official recognition of Minamata disease, Japan's worst industrial pollution-induced disease caused by methyl mercury contained in wastewater released into the Yatsushiro Sea from the Minamata factory of chemicals maker Chisso Corp.
On May 1, 2010, Prime Minister Yukio Hatoyama became the first prime minister to attend an annual memorial service for the Minamata disease victims held in the city and apologized for the government's failure to prevent both the spread of the industrial pollution and the outbreak of the neurological disease.
The organic mercury poisoning affected residents in eastern coastal areas of Kumamoto and Kagoshima prefectures and 2,271 people in the areas have been officially recognized as Minamata disease sufferers. In 1965 similar symptoms were found among residents along the Agano River in Niigata Prefecture — the first recognition of Daini (Second) Minamata disease, which was found to be caused by wastewater from a chemicals factory of Showa Denko K.K.