The contents of polychlorinated biphenyls (PCBs) in the blood of Yusho patients (Y) and normal subjects (N) were analyzed, using a gas-chromatograph equipped with a capillary column/quadruple mass spectrometer (capillary column GC/MS). While the average of the content of blood PCBs was 679 ppt in N, those were 2960, 1480 and 1090 ppt in Y diagnosed previously as A, B and C patterns, respectively. The contents of non-ortho coplanar PCBs (3,3',4,4' Tetra-, 3,3',4,4',5-Penta- plus 3,3',4,4',5,5'-Hexa-CB) were 0.9-2.1 ppt in Y, and 0.8 ppt in N. The contents of eight mono-ortho coplanar PCBs having the toxic equivalency factors (TEFs) were 118-424 (Y) and 78 ppt (N), respectively. The differences between Y and N in the species of mono-ortho coplanar PCBs contained were observed as follows: 2,3,3',4,4',5- and 2,3,3',4,4',5'-Hexa-CB were predominant in Y, whereas 2,3,4,4',5-Penta-CB was a major congener in N. The levels of di-ortho coplanar PCBs (2,2',3,4,4',5,5' plus 2,2',3,3',4,4'5-Hepta-CB) were determined to be 195-608 and 98 ppt in Y and N, respectively. In both groups, the content of the former isomer was greater than the latter. Total TEQ values were 0.214-1.226 ppt in Y, and 0.148 ppt in N. The analyses were also performed by a conventional method in which a GC equipped with a packed column/electron capture detector (packed column GC/ECD) was used, and the results obtained were compared with those by capillary column GC/MS method. The results showed that the total PCB levels obtained by capillary column GC/MS method were 56-61% in Y and 67% in N of those obtained by packed column GC/ECD method.