Inherited thrombophilic states are associated with an increased risk for deep vein thrombosis (DVT), but whether they are also risk factors for superficial vein thrombosis (SVT) is uncertain. We assessed the risk conferred by inherited thrombophilic states ill patients with. a first episode of SVT in whom the coexistence of DVT had been ruled out by ultrasonography. Sixty-three patients with SVT, after exclusion of patients with varicose veins, malignant or autoinimmune disease, and 537 healthy individuals were investigated. The G1691A mutation in the factor V gene, the G20210A mutation in the prothrombin gene, and deficiencies of the naturally occurring inhibitors of coagulation (antithrombin, protein C, protein S) were searched. The prevalence of each thrombophilic state was higher in patients than in controls. The odds ratios for SVT were 6.1 (95% confidence interval [CI], 2.6 to 14.2) in patients with the G1691A factor V mutation, 4.3 (95% CI, 1.5 to 12.6) in those with the G20210A prothrombin mutation, and 12.9 (95% CI, 3.6 to 46.2) in those with deficiencies of the naturally occurring inhibitors of coagulation taken together. Risks did not substantially change when the analysis was restricted to 43 patients who had SVT as their only thrombotic manifestation, being 4.3 (95% CI, 1.5 to 12.3) in patients with factor V mutation, and 3.6 (95% CI, 1.0 to 13.1) in those with the prothrombin mutation. Among the circumstantial risk factors investigated (surgery, trauma, prolonged immobilization, oral contraceptives and pregnancy or puerperium, pregnancy or puerperium was the most frequently associated with SVT, being present in 38% of women. Our findings indicate that inherited thrombophilic states are associated with an increased risk for SVT. Hence, a laboratory search of these alterations is recommended in patients with SVT, because it allows the identification of patients at high risk of DVT in whom antithrombotic prophylaxis is particularly warranted.