2'-Deoxyoxanosine (dOxo) is a novel DNA lesion produced from 2'-deoxyguanosine by the treatment with nitrous acid and nitric oxide. However, there has been no direct evidence on the formation of dOxo in vivo so far. In order to detect dOxo in vivo, we have explored a sensitive detection method specific for dOxo by using a fluorescent labeling reagent and HPLC analysis. An efficient method to detect the dOxo formation in DNA is described.