The occurrence and progress of Yusho (PCB mass poisoning) is briefly described. Various investigative studies on Yusho are also briefly reviewed. Yusho occurred in the northern district of Kyushu Island in 1968. A specific product of rice oil used for food was accidentally contaminated with PCB during its manufacturing process and consequently, the customers who ate it showed severe intoxication symptoms. Major clinical symptoms of Yusho were acneiform eruptions and circumscribed pigmentation. These skin and mucous membrane eruptions, because of their appearance, caused severe mental suffering for the patients. The government-authorized patients amounted to 1,860. These symptoms have gradually declined with the passing of years and presently the principal clinical problems are only various subjective complaints. At first, it was naturally thought that the causative agent of Yusho was PCB (Kanechlor 400) itself. However, it was found later that the causative rice oil contained PCDF (polychlorodibenzofuran) and PCQ (polychloroquaterphenyl) which were proved to be formed during the heating process used in the deodorization of the oil. PCDFs show, in animal studies, a high toxicity and therefore its role for the pathogenesis of Yusho is seriously evaluated in spite of its rather small contaminating amount. On the other hand, PCQs are recognized as the specific marker of Yusho, since almost no PCQ is a general environmental pollution substance.