AIM: The purpose of this study was to investigate possible effects of green tea extract on the activities of DNA turn-over enzymes, namely adenosine deaminase (ADA) and xanthine oxidase (XO) in gastric and colon tissues from patients with stomach and colon cancer. MATERIALS AND METHODS: Six cancerous and 6 non-cancerous adjacent human gastric tissues, and 7 cancerous and 7 non-cancerous adjacent colon tissues obtained surgically were treated with aqueous green tea extract at 3 different concentrations for 1 hour, and then ADA and XO activities were measured. RESULTS: In all of the tissues, XO activities were found to elevate after treatment with green tea extract. Additionally, ADA activity was found to be inhibited in the cancerous gastric tissues by the green tea extract. Elevated XO and reduced ADA activities due to treatment with green tea extract may lower salvage pathway activity and lead to inhibition in carcinogenesis. CONCLUSION: Our data suggest that green tea may support the medical treatment of stomach and colon cancer.