The role of oxidative stress in tobacco smoke damage was examined using a combination of in vitro and in vivo approaches. Aqueous extracts of cigarette smoke were irradiated with UV and hydroxyl radical generation was evaluated by electron spin resonance (ESR). The spectra obtained revealed spin adducts of the hydroxyl radical (*OH) to increase with the volume of the aqueous extract of cigarette smoke, cigarette smoke collection flow, and UV irradiation time. Hydroxyl radical generation persisted for long hours, showing no change over time. Ascorbic acid protected against radical generation in a marked concentration-dependent fashion. In specimens of urine from volunteers, 8-OHdG levels and the 8-OHdG-production rate per hour were found to be higher in smokers than non-smokers. Our results suggest that more marked oxidative stress occurs with the smoking habit.