A pulmonary embolism is a blockage or closure caused by a clot of an artery in the lung. This short-term complication is potentially life-threatening and occurs in about 10% of patients with acute DVT events. It may be even more common, since up to 75% of embolisms are without symptoms. Routine diagnostic testing is necessary to discover whether pulmonary embolism is present.

This late complication of DVT is the most common—occurring in up to two-thirds of patients. Possible signs and symptoms are pain, edema (accumulation of fluid), hyperpigmentation (increase in skin color), and skin ulceration. Severe manifestations and ulceration occur in 7–23% and 4–6%, respectively, of people with DVT.

Post-thrombotic syndrome may result from some obstructions that remain in the vein or from reflux (backflow of blood) or both (more likely). Obstruction may in turn contribute to reflux. Rate of reflux is highest during the 6 to 12 months after acute DVT. It may occur transiently in up to 23% of DVT patients and may resolve during the follow-up period.

The extent of the thrombus may have some part in causing post-thrombotic syndrome, but other factors, too, may be determinants. These are rate of recanalization (formation of new pathways or blood vessels), recurrent thrombotic events, extent of reflux, and venous valve function.