Recently, it has become apparent that superficial vein thrombosis (SVT) can have serious complications. However, the magnitude of the risk of subsequent deep venous and arterial thrombotic events remains unknown. We examined this in a nationwide population-based setting during a period when SVT was not treated routinely with anticoagulants. The Danish National Registry of Patients, covering all Danish hospitals, was used to identify 10 973 patients with a first-time diagnosis of SVT between 1980 and 2012. A comparison cohort of 515 067 subjects, matched by age, gender, and calendar year, was selected from the general Danish population. Outcomes were venous thromboembolism, acute myocardial infarction, ischemic stroke, and death. During median follow-up of 7 years, the incidence rate of venous thromboembolism was 18.0/1000 person-years (95% confidence interval [CI], 17.2-18.9). The highest risk occurred in the first 3 months (3.4%; 95% CI, 3.0-3.7). Compared with the general population, the hazard ratio was 71.4 (95% CI, 60.2-84.7) in this period, steadily decreasing to 5.1 (95% CI 4.6-5.5), 5 years after the SVT. The hazard ratios for acute myocardial infarction, stroke, and death were 1.2 (95% CI, 1.1-1.3), 1.3 (95% CI, 1.2-1.4), and 1.3 (95% CI, 1.2-1.3), respectively, with the highest risk also shortly after SVT. These data indicate the prognostic importance of SVT and may form the basis for clinical decision-making regarding anticoagulation.