Objective: The aim of this study was to examine Factor V G1691A (Leiden) (FVL) and prothrombin G20210A (PT) gene mutation status, and their relationship with thrombosis in patients with chronic myeloproliferative disorders (CMPDs). Materials and Methods: The study included 160 patients with a CMPD that were regularly followed-up between 1993 and 2009. FVL and PT mutation status was established based on blood samples analyzed via PCR using specific primers. Results: The frequency of FVL and PT mutation was 12.5% and 4.4%, respectively. In total, 27 episodes of thrombosis occurred in 24 (15%) of the patients, and there wasn't an association between the observed thrombotic events, and FVL or PT mutations. Hepatic vein thrombosis was noted in 3 patients that had FVL mutation, of which 1 also had PT mutation. Conclusion: We did not observe a relationship between thrombosis, and FVL or PT mutations in CMPD patients; however, 3 of the patients that had hepatic vein thrombosis also had FVL mutation. Larger studies are needed to more clearly determine if all CMPD patients with hepatic vein thrombosis need be investigated for FVL and PT mutation. (Turk J Hematol 2011; 28: 306-11)