Reassessment of dibenzo-p-dioxins and dibenzofurans (PCDD/DFs) and coplanar polychlorinated biphenyls (Co-PCBs) in toxic Yu-Cheng rice-bran oil was carried out. The contaminating source appears to be a mixture of KC-400 and KC-500. The oil contains 176 micrograms/g PCBs and 37.194 ng/g 2,3,7,8,-TCDD toxicity equivalency (TEQ). The TEQ contributions from the PCDDs (4.472 ng/g), PCDFs (19.568 ng/g), and Co-PCBs (13.154 ng/g) are 12% 53% and 35%, respectively. Complete identification and quantitation suggests that 2,3,4,7,8-PeCDF, 1,2,3,4,7,8-HxCDF, 2,3,7,8,-TCDF. 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD and PCB# 77 are candidates for the primary causal agents of Yu-Cheng disease.