Factor V (FV) Leiden mutation is responsible for the development of inherited activated protein C (APC) resistance in the majority of cases and cancer is an acquired risk factor for thrombosis. The aim of this sudy is to investigate the factor V Leiden mutation frequency in patients with lung cancer. Screening for the FV Leiden mutation in 44 patients with lung cancer was performed using polymerase chain reaction amplification of exon 10 followed by Mnl I restriction enzyme digestion and agarose gel electrophoresis. Results are as follows: 44 lung cancer patients were included in the study (40 males, 4 females, age range 42-87, median age 60); 34 (77%) had nonsmall cell lung cancer (NSCLC), 9 (21%) had small cell lung cancer (SCLC) and one had a combined carcinoma. Six of them were nonsmokers while the others were heavy smokers and they had no history of previous thromboembolic events. Only one patient was heterozygous and none of them was homozygous for FV Leiden mutation. Factor V Leiden mutation frequency was not increased in patients with lung cancer. However further investigations should be undertaken to assess the interaction of genetic risk factors and cancer. It would be beneficial to define the risk rates for cancer patients receiving anticoagulant therapy who are carriers of a genetic risk factor and have no genetic risk factor for thrombosis.