AbstractThrough the analysis of polychlorinated biphenyls (PCBs), polychlorinated quaterphenyls (PCQs), and polychlorinated dibenzofurans (PCDFs) retained in the blood of Japanese and Taiwanese patients who had been poisoned by ingesting contaminated cooking oils, in the blood of workers occupationally exposed to PCBs, and in the blood of unexposed individuals, it was strongly suggested that PCDFs might be the responsible compounds for the development of Yusho disease. To confirm this, animal experiments were done. The dermal signs resembling those of Yusho were observed among the cynomolgus monkeys that received a PCDF mixture consisting of PCDF components similar to those in the Yusho oils. In addition, significant thymic atrophy and decreased body weight gain were noted in the rats fed PCDFs. However, the administration of PCBs or PCQs alone failed to cause such signs in the monkeys or rats. As a result, it became clear that PCDFs were the main causative agents in the pathogenesis of Yusho disease.