OBJECTIVE To evaluate the occurrence of deep vein thrombosis (DVT) in patients with superficial vein thrombosis (SVT). METHODS A prospective study in patients with sonographically proven SVT. METHODS Outpatient department of the Department of Dermatology, Medical University of Graz. Patients Forty-six consecutive patients with superficial vein thrombosis were enrolled. Intervention Every patient underwent color-coded duplex sonography of both lower extremities at the beginning of the study. METHODS Important risk factors (eg, history of thromboembolic events, recent immobilization, active malignant disease, and the use of oral contraceptives) were investigated. RESULTS In 24% of our patients, a concomitant, mostly asymptomatic DVT was found. In 73% of these patients, the DVT occurred in the affected leg, in 9% in the contralateral leg, and in 18% in both legs. The calf muscle veins were most commonly involved. In all patients with DVT, the SVT was located on the lower leg and the D-dimer findings were positive. CONCLUSIONS Superficial vein thrombosis is not a life-threatening disease, but the risk of concomitant DVT cannot be ignored. Color-coded duplex sonography should be performed in patients with SVT to rule out DVT.