Single cell gel electrophoresis assay (SCGE), also named as alkaline comet assay, was a simple, rapid and sensitive method to evaluate DNA damage. In this study SCGE technique was used to monitor DNA damage difference in tumor patients caused by chemotherapy, DNA damage distribution frequency and DNA damage characters were analyzed by komet image analysis system (KIAS). The results showed that cyclophosphamide greatly caused DNA damage in lymphocytes of tumor patients. There was significant difference of peripheral blood lymphocyte DNA damage between tumor patients and healthy controls. Tail length of lymphocytes were 33.69 +/- 7.56 micro m, and tail DNA% we re 31.51 +/- 5.4 6% in 10 cancer patients treated with cyclophosphamide, while Tail length were 1 6.2 +/- 1.5 micro m and tail DNA% were 7.46 +/- 1.15% in healthy controls. there was great significant difference on tail length and tail DNA% values between cancer patients and healthy controls (P < 0.01). In conclusion, the successful measurement of DNA damage caused by Cyclophosphamide treatment means that the alkaline comet assay as a valuable tool can be very useful in cancer epideminology study, and also be valuable to evaluate DNA damage status of patients in clinic.