Although superficial vein thrombosis is commonly considered a rather minor condition, a number of studies indicate that its consequences can be much more severe. Since the introduction of Doppler ultrasonography to common diagnosis of venous diseases, the approach to threats associated with superficial vein thrombosis has changed, mainly in the context of venous thromboembolism. Superficial thrombosis in varicose veins must be differentiated from that occurring in patients without varicosities. In the former case, superficial vein thrombosis is usually caused by haemodynamic disorders (slower flow), while in the latter, it is caused by thrombophilia or inflammation, but it can also be a prodromal sign of cancer. Ultrasonography enables one to distinguish deep vein thrombosis caused by superficial vein thrombosis progression (by extension through perforator veins or the great/small saphenous vein ostium into the deep venous system) from deep vein thrombosis occurring at a certain distance from the site of superficial vein thrombosis. The authors emphasise that due to the possibility of concomitant deep vein thrombosis or pulmonary embolism, our attitude to the diagnostic process, potential complications and treatment of superficial vein thrombosis should be changed.