Breast milk and blood of Yusho patients were analyzed for polychlorinated biphenyl (PCB) congeners by High separation gas chromatography/Low resolution mass spectrometer. Seventy-one and forty-nine PCB congeners were identified and quantified in the breast milk and blood, respectively. Total PCB concentrations (Whole base) in breast milk of 2 Yusho patients were 69.9 and 15.1 ppb, respectively, being 11.6 and 2.5 times higher than those of 4 control breast milk. Average total PCB concentration of 5.0 ppb in whole blood of 13 Yusho patients was 3.6 times higher than those of control persons. Fourteen PCB congeners, such as 99, 117, 130, 137, 138, 156, 157, 164, 170, 171, 172, 189, 191 and 195, were particularly higher concentration in breast milk and blood of Yusho patients than in those of control persons. Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and coplanar PCBs were also analyzed in the breast milk of Yusho patients and control persons. Large portions, 83 and 74%, of Dioxin toxic equivalency (TEQ) in the 2 Yusho breast milk were consisted of TEQ of 2,3,4,7,8-pentaCDF only, while total TEQ in the breast milk of control persons was consisted of PCDFs 48%. PCDDs 29% and coplanar PCBs 23%.