Symptoms include swelling, pain and reddening of leg, but not all patients experience symptoms.
The most serious complication is a pulmonary embolism, where part of a clot breaks off and travels to the lungs, where it can then block key arteries. This can be fatal.
Many DVT clots are relatively small and the body can break them down with no long-term effects. They can occur for no apparent reason but are associated with periods of inactivity - such as sitting on long-haul flights or staying in bed to recover from surgery.