Thyroid tumours are the most common endocrine malignancy, accounting for approximately 1% of all malignant diseases and about 0.4% of deaths related to cancer. Several studies have reported increased thyroid, breast and bladder cancer risks associated with higher DNA damage measured by the comet assay. The aim of the present study was to investigate the possible DNA damage in the peripheral blood lymphocytes patients with benign and malignant thyroid disorders in comparison to healthy controls by using the comet assay. In this study, DNA damage expressed as tail length, tail intensity and tail moment of 45 patients with benign and 26 patients with malignant thyroid disorders operated in Gazi University, Medical Faculty, Department of Surgery were compared with 30 healthy controls comparable in sex, socio-economic life style, and smoking habits with no history of any thyroid disease. Patients with benign and malignant thyroid disorders were not received chemotherapy and radiotherapy. Significant increases were observed in the DNA damage in patients with benign and malignant thyroid disorders compared to the healthy controls. No significant differences were found in the DNA damage between benign and malignant thyroid disorders.