We investigated DNA base damage in genomic DNA of lymphocytes of cancer patients undergoing radiation therapy. Lymphocyte chromatin samples were analyzed by gas chromatography/isotope-dilution mass spectrometry for DNA base damage. The results provided evidence for formation of typical hydroxyl radical-induced base modifications in genomic DNA of lymphocytes. Different levels of DNA products in individuals were observed and, in the case of some patients, there was no significant product formation, possibly resulting from differences between individuals and between the types of radiation exposures. Decreases in product levels after an initial increase by radiation exposure were observed. This may indicate the removal of modified bases from lymphocyte DNA by cellular repair.