DNA damage has been implicated in carcinogenesis and aging, and is believed to underlie the effectiveness of many anticancer drugs. DNA damage can arise from attack by reactive oxygen and nitrogen species generated during normal metabolism and inflammation, as well as following exposure to radiation, pollutants, and pharmacologic agents. Extensive DNA damage can induce apoptosis and, if widespread, may impair organ function. While limited DNA damage can be repaired, imperfect repair may produce mutations contributing to cellular transformation or a reduction in cellular function. To investigate this process, we have optimized a flow cytometric assay to measure DNA damage and repair kinetics as a function of cell cycle position. We have applied the assay to cell lines treated with agents known to cause oxidative damage to DNA (Carcinogenesis (2002) 23:101). We are currently investigating cell cycle phase-dependent DNA damage, resistance and repair in proliferating cells treated in vitro with topoisomerase-inhibiting cancer chemotherapeutics or alkylating agents. This work has included the use of cell lines with known DNA repair deficiencies (e.g., Werner’s syndrome, ataxia-telangiectasia) and cell lines resistant to chemotherapy (e.g., topoisomerase II deficiencies). Future investigations may include analysis of the efficacy of prospective cancer chemotherapeutics, the capabilities of antioxidants to protect DNA from damage, and the determination of the extent of DNA damage induced in blood tumor cells following clinical treatment.