Thrombosis occurs most often as myocardial infarction, cerebral infarction or venous thromboembolism, ie, deep-vein thrombosis and pulmonary embolism. The incidence of all types of thrombosis is strongly dependent on age. Among young individuals, up to age 40, venous thrombosis is the most common form of thrombosis. The risk factors for arterial and venous thrombosis differ, and among the latter disorders of hemostasis appear to be more prominent. In children venous thrombosis appears almost exclusively in association with venous catheters, with an exception of the renal vein thrombosis of the newborn, which has an unknown etiology. In young adults, the risk factors for venous thrombosis are essentially the same as in older individuals, excepting oral contraceptives, pregnancy and puerperium which are limited to young women. In young women, most venous thrombotic events can be attributed to oral contraceptives. Venous thrombosis is a multicausal disease: more than one risk factor needs to be present before thrombosis occurs. The younger an individual, the more risk factors are required to precipitate thrombosis: in children often three or four, and in young adults often two or more.