Oxidative stress reflects a disturbance in the balance between the production and accumulation of reactive oxygen species (ROS). When ROS levels are elevated, oxidative damage occurs in proteins, lipids, and DNA. 8-hydroxy-2-deoxyguanosine (8-OHdG) is a product of oxidative DNA modifications; therefore, urinary 8-OHdG serves as a non-invasive biomarker of oxidative DNA damage. In this context, we developed an analytical method for the quantification of urinary 8-OHdG based on LC-MS/MS.