This study was designed to evaluate the possible in vivo induction of DNA damage by exposure to radiation in cardiologists. The alkaline cornet assay (single cell gel electrophoresis, SCGE), which appears to be a promising tool with which to estimate DNA damage at the single cell level, has been used. The assay was carried out on 30 cardiologists currently employed in a busy cardiac service and 30 healthy unexposed controls. Venous blood samples were obtained from the exposed and control subjects and SCGE was examined in 100 cells graded as undamaged, intermediate, and tailed nuclei. The number of undamaged nuclei was almost the same in control and exposed Subjects. The extent of DNA migration (SCGE assay) did not distinguish between the samples in either the nonsmoker exposed or nonsmoker control subjects, which leads one to wonder whether a difference in DNA damage really exists. Previous studies reported increased DNA damage in blood lymphocytes of smokers.