Venous thrombosis (the formation, development, or existence of a blood clot in a vein) is common and carries a substantial risk for long-term suffering or death. Deep vein thromboses that break away from the vein, flow to the lungs, and block circulation (pulmonary emboli) cause over 1,000 deaths annually in Sweden and are the single most common cause of death during pregnancy and in surgical intervention where prophylaxis is not used. The risk for this disorder increases with age. At older ages it is more common in men; at younger ages it is more common in women. Annually, over 11,000 people are hospitalized for venous thrombosis or pulmonary embolism, and approximately 40,000 patients visit a physician in ambulatory care. In a longer perspective, many people are affected by leg swelling, varicose veins, and slowly healing leg ulcers (conditions not addressed in this report).