The role of age, gender and smoking on both the genotoxic effects of Helicobacter pylori and the efficacy of eradication therapy in a group of patients with gastritis was investigated. Gastritis was confirmed by endoscopy and biopsy, and the presence of H. pylori by urease testing. Pre- and post-treatment peripheral blood lymphocyte cultures were prepared from 17 patients and 25 metaphases per patients were analysed for sister chromatid exchange (SCE), a well-established technique for the evaluation of human exposure to toxic agents. Treatment with omeprazole clarithromycin and amoxycillin triple therapy eradicated H. pylori in 94% of patients and significantly reduced the SCE frequency. Pre-treatment SCE frequency was found to be positively correlated with age. Female smokers tended to have higher post-treatment SCE frequencies than male smokers, and pre- and post-treatment SCE frequencies were higher in older males than in older females. Eradication therapy decreased the genotoxicity of H. pylori, but age in males and smoking in females may decrease treatment efficacy.