Two mass-food poisonings called Yusho and Yucheng occurred in Japan and Taiwan in 1968 and 1979, respectively. Blood samples from five Yusho patients and three Yucheng patients were collected several times from 1982 to 1998 and from 1980 to 1995, respectively, and analyzed for polychlorinated dibenzofurans (PCDF) and polychlorinated biphenyls (PCB) congeners. Fat base concentrations of 2,3,4,7,8-penta-CDF in the Yusho patients, which were responsible for about 70% of the dioxin toxicity, are estimated to be decreased from 60 ppb in 1969 to 800 ppt in 1997 with the median half-life of 2.9 years in the first 15 years after onset and 7.7 years in the next stage of 15 years. Typical Yusho symptoms of acneiform eruption, dermal pigmentation and increased eye discharge were very gradually recovered with lapse of several years. However, enzyme and/or hormone-mediated signs of high serum triglyceride, high serum thyroxin, immunoglobulin disorder and others are persistently maintained for 30 years.