Recently, H. Kasai reported an automatic, precise method of 8-hydroxydeoxyguanosine (8-OH-dG) analysis in urine by high performance liquid chromatography coupled to an electrochemical detector (HPLC-ECD). It is based on a cleaning-up step by anion-exchange chromatography and a further purification step using reverse phase chromatography before detection, by the ECD. In this communication, we report a method for the simultaneous determination of 8-OH-dG and creatinine, an internal standard for normalizing the excretion of 8-OH-dG in urine.