To assess whether Helicobacter pylori-related inflammation increases oxidative DNA damage, we evaluated the association between H. pylori infection and urinary excretion of an adduct of oxidative DNA damage, 8-hydroxy-2-deoxyguanosine (8ohdG). Subjects included 555 healthy persons, ages 20-39, within the Kaiser Permanente Medical Care Program in Northern California. We tested sera for antibodies to H. pylori by ELISA; collected demographic, dietary, smoking, and alcohol data by questionnaire; and assayed 24-h urine samples for 8ohdG with a newly developed ELISA kit. Two hundred eighty-one subjects provided adequate 24-h urine samples for 8ohdG and creatinine assays and had detectable levels of 8ohdG. After adjusting for 24-h urinary creatinine (Ucr) and demographic factors, persons without H. pylori infection had significantly higher amounts of 24-h urinary 8ohdG than infected persons (geometric mean, 18.04 microg 8ohdG/Ucr g versus 14.36 microg 8ohdG/Ucr g, respectively; P = 0.008). Excretion of 8ohdG was higher in whites and Hispanics (17.44 and 18.09 microl/Ucr g) than in blacks (13.21 microg/Ucr g; P < 0.001). Gender was not significantly associated with 8ohdG excretion (16.18 microg/Ucr g for males versus 16.01 microg/Ucr g for females; P = 0.883). Of the dietary factors evaluated, vitamin C negatively correlated (P < 0.001) and carbohydrate intake positively correlated with 8ohdG excretion (P = 0.003). Infection with H. pylori was strongly associated with decreased 8ohdG excretion in the urine. This unexpected finding suggests either that DNA repair is deficient in infected subjects, that inflammation destroys the adduct, or that urinary 8ohdG is not an accurate measure of gastric damage.