Urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG) is considered to be a biomarker of cellular oxidative stress and to be associated with carcinogenesis. To estimate the oxidative stress caused by radiotherapy, we used the enzyme-linked immunosorbent assay (ELISA) to monitor urinary 8-OHdG levels in 72 patients undergoing radiotherapy. Subjects consisted of 23 breast cancer patients undergoing post-operative external radiotherapy for breast conserving therapy, 12 tongue and seven prostate cancer patients undergoing interstitial radiotherapy, 19 esophageal cancer patients undergoing external radiotherapy and chemotherapy, and 11 cervical cancer patients undergoing external radiotherapy and brachytherapy with or without chemotherapy. Before radiotherapy, cervical cancer patients showed a higher urinary 8-OHdG level (16.0+/-8.8 ng/mg) than breast cancer patients (5.3+/-5.5 ng/mg, p=0.001). In the other three groups, urinary 8-OHdG levels were in the medium range (prostate cancer patients: 6.6+/-6.3 ng/mg; esophageal cancer patients: 8.8+/-11.1 ng/mg; tongue cancer patients: 10.2+/-6.7 ng/mg). Radiotherapy did not cause changes in the excretion level of urinary 8-OHdG in patients with breast, esophageal and tongue cancer. However, radiotherapy reduced 8-OHdG excretion levels in patients with cervical cancer, whereas interstitial radiotherapy transiently increased these levels in patients with prostate cancer.