Ischaemia-reperfusion (I/R) injury is responsible for a number of conditions such as coronary bypass and myocardial infarction, and deaths. Oxygen-free radicals formed during I/R have been proposed as the leading causes of tissue injury, and they play an important role in I/R injury. I/R induces oxidative DNA damage (such as purinic and pyrimidinic base lesions). Comet assay is a suitable and sensitive method for early detection of low-level DNA damage. We used modified alkaline comet assay in peripheral blood lymphocytes and evaluated I/R-induced DNA damage in patients undergoing coronary artery bypass graft (CABG) operation (in vivo model for I/R). No statistically significant difference in DNA damage levels was found before surgery, after anaesthesia, ischemia, reperfusion, and surgery. However, blood lactate levels (assessed in parallel with the comet assay) increased after I/R and did not return to the baseline level. Our findings showed that I/R injury did not induce DNA damage, but increased the lactate levels. This finding suggests that there might be reversible and uncommon necrosis that did not reflect on overall DNA base damage. Further studies are needed using this approach.