Polychlorinated biphenyls (PCBs) are known to cause the release of superoxide during the metabolic process. Therefore, it is suggested that the Yusho patients are exposed to oxidative stress caused by high concentrations of PCBs are still recorded in their serum. Recently, diacetylspermine (DiAcSpm) has been proposed as tumor markers or barometer of oxidative stresses. In order to estimate the ability of urinary DiAcSpm as a potential marker of dioxin or PCB exposure, we measured urinary DiAcSpm in Yusho patients and assessed the association of DiAcSpm with serum PCBs and dioxins levels. The statistical analysis did not indicat that urinary DiAcSpm is useful for detection of exposure to PCBs or dioxins, although a few Yusho patients showed high level of DiAcSpm.