Minamata disease occurred in the vicinity of Minamata Bay, Japan, from eating a large quantity of fish contaminated with wastewater discharged from a chemical plant. The disease is the same as methylmercury poisoning and annoyed many people with neurological disorders such as ataxia, speech disturbance, and constriction of visual fields. The process of verifying the causal agent was the history itself. The number of patients with the disease could have been lowered if the Government of Japan had applied the precautionary principle to this epidemic by way of identifying the causal agent. On the other hand, the discovery of fetal Minamata disease provided to later investigators new evidence that intrauterine exposure to the chemical affects offspring, and advanced today’s birth cohort studies. At present, since some developing countries appear to lie in similar situations to Japan in the past, attention should be directed to early recognition of a risky agent and precautions against it.