Concentrations of polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), coplanar polychlorinated biphenyls (Co-PCBs) and total polychlorinated biphenyls (PCBs) were analyzed in blood samples from 83 Yusho patients and 39 normal control subjects in 1995. The average concentrations of PCDDs, PCDFs and Co-PCBs in the blood of Yusho patients were 14, 128 and 14 pg toxic equivalent quantity (TEQ)/g, lipid, respectively. The levels of these compounds in the blood of normal subjects were 14, 10 and 8 pg/g lipid, respectively. Thus, although the average levels of PCDDs and Co-PCBs in the blood of Yusho patients were similar to those of control subjects, the PCDF level in Yusho patients was 12.8 fold higher than in normals. Average concentrations of total PCBs in the blood of Yusho patients and normal control subjects were 789 and 339 ng/g lipid, respectively. Thus, the PCB level in the blood of Yusho patients was 2.3 fold higher than in normal subjects. To test the reproducibility of determination of PCDDs, PCDFs and Co-PCBs, three normal blood samples were analyzed twice. These analyses gave similar results each time. Since, these compounds could be measured in five-milliliter blood samples, they can be monitored in Yusho patients.