Factor V (FV) plays a crucial role in both procoagulant and anticoagulant pathways. It was indicated that -426G/A change in the promoter of the FV gene may have an effect on the occurrence of thrombosis. We aimed to investigate the effect of -426G/A gene variation on thrombus formation in the thrombosis patients of age 0 and 18 years and 70 years and older. The study included 179 patients with the diagnosis of thromboembolism and also 221 healthy individuals as controls. The Polymerase chain reaction–restriction fragment length polymorphism method was used to detect -426G/A. It was observed that carrying AA genotype does not have a significant risk of thrombosis between the patients and controls (for 0-18 age group P = .96, odds ratio [OR]: 1.13 [0.38-3.30], for ≥70 age P = .94, OR: 0.69 [0.11-4.28]). The same results were obtained when FV Leiden mutation carriers were excluded (for 0-18 age group P = .95, OR: 1.15 [0.35-3.78], for ≥70 age P = .89, OR: 0.73 [0.11-4.56]). In this study, an effect on thrombosis of -426G/A polymorphism was not determined.