Venous thrombosis which mainly manifests as deep vein thrombosis of the leg or pulmonary embolism occurs in 1 per 1000 per year. It occurs due to interacting genetic, environmental and behavioral risk factors. The strongest risk factors are certain types of surgery and malignancies. Over the last decade many new risk factors for venous thrombosis have been identified. Venous thrombosis has a high recurrence rate, of around 5 % per year. Whereas clinically it would be most important to identify patients at risk of recurrence, only male sex and a previous unprovoked thrombosis are established determinants of recurrent thrombosis.