The single cell gel electrophoresis technique (comet assay) has been adopted to demonstrate the DNA damage in human leukocytes of individuals who are habitual smokers. This technique allows the detection of single strand and double strand DNA breaks, which are indicative of the risk of cancer. The method followed is rapid, inexpensive, and a highly sensitive technique for the evaluation of DNA damage and risk assessment in smokers. This experiment should help students understand the effect of cigarette smoking.