Polychlorinated biphenyl (PCB) poisonings occurred in western Japan, where it is called yusho, in 1968, and in central Taiwan, where it is called yu-cheng, in 1979. The average concentrations of PCBs in the adipose tissue, liver and blood of yusho patients and in the blood of yu-cheng patients were 1.9 ppm, 0.08 ppm, 6.7 ppb and 99 ppb, respectively. Seven PCB congeners, such as 2,4,5,3',4'-pentachloro-, 2,3,4,3',4'-pentachloro-, 2,4,5,2',4',5'-hexachloro-, 2,3,4,2',4',5'-hexachloro-, 2,3,4,5,3',4'-hexachloro-, 2,3,4,5,2',4',5'-heptachloro- and 2,3,4,5,2',3',4'-heptachloro biphenyls were identified in the blood and tissues of patients with yusho and yu-cheng and controls. The concentration of 2,3,4,5,3',4'-hexachlorobiphenyl was comparatively higher in the patients than in controls. The concentrations of polychlorinated dibenzofurans (PCDFs) in the adipose tissue and liver of yusho patients were 6 to 13 ppb and 3 to 25 ppb, respectively, while no PCDFs were detected in the controls. Major PCDF congeners identified in the tissues and blood of yusho and yu-cheng patients were the 2,3,6,8-tetrachloro-, 2,3,7,8-tetrachloro-, 1,2,4,7,8-pentachloro-, 2,3,4,7,8-pentachloro- and 1,2,3,4,7,8-hexachlorodibenzofurans (DFs), of which the 2,3,4,7,8-pentachloro compound was predominant. The concentrations of methylthio-PCB in the liver, lung and adipose tissue of yusho patients were 0.1 to 0.5, 0.2 to 1.4 and 0.5 to 1.0 ppb, respectively, and those of methysulfone PCBs were 0.3 to 0.7, 1.0 to 2.5 and 0.7 to 1.0 ppb, respectively. Some of the major peaks of the PCB methylthio and methylsulfone derivatives were identical in gas chromatographic retention times with those of 4-methylthio- and 4-methylsulfone-2,5,2',5'-tetrachlorobiphenyl PCDFs, especially 2,3,4,7,8-pentachloro DF, appear to be mainly responsible in the poisonings.