A total of 151 umbilical cords during the period from 1950 to 1969 were collected from the residents of the Minamata area (including 25 patients with congenital Minamata disease) for methylmercury (MeHg) analysis. When the MeHg discharge from the Chisso Company's Minamata factory into the Minamata Bay is compared with the incidence of congenital Minamata disease, the abrupt increase of the former in 1952 [Nishimura H. Chem. Today 1998;323:60-66] was found to precede that of the latter by approximately 2 years, thereby indicating that MeHg is the cause of the disaster. This was confirmed by the elevated levels of MeHg in the umbilical cords from residents of the Minamata area [from 0.35 +/- 0.30 (S.D.) ppm in 1952 to 0.96 +/- 0.75 ppm in 1955], the MeHg levels (1.60 +/- 1.00 ppm) in the cords from patients with congenital Minamata disease showing the highest values [P < 0.01 vs. acquired Minamata disease (0.72 +/- 0.65 ppm), mental retardation (0.74 +/- 0.64 ppm), other diseases (0.22 +/- 0.15 ppm), and no symptoms (0.28 +/- 0.20 ppm), respectively]. Thus, in order to fill a gap, which extends over a long period of time, in studies on environmental Hg pollution, umbilical cord samples were considered to be a useful tool.