Parkes Weber syndrome (PWS) is a rare congenital condition characterized by a large number of abnormal blood vessels. The main signs and symptoms of PWS typically include a capillary malformation on the skin; hypertrophy (excessive growth) of the bone and soft tissue of the affected limb; and multiple arteriovenous fistulas (abnormal connections between arteries and veins) which can potentially lead to heart failure. There also may be pain in the affected limb and a difference in size between the limbs.

Some cases of Parkes Weber syndrome result from mutations in the RASA1 gene, and are inherited in an autosomal dominant manner. In these cases, affected people usually have multiple capillary malformations. People with PWS without multiple capillary malformations are unlikely to have mutations in the RASA1 gene; in these cases, the cause of the condition is often unknown. Management typically depends on the presence and severity of symptoms and may include embolization or surgery in the affected limb.

The signs and symptoms of Parkes Weber syndrome include:

• Birthmarks caused by capillary malformations on the skin; they increase blood flow near the surface of the skin and usually look like large, flat, pink stains on the skin, and because of their color are sometimes called "port-wine stains."
• Hypertrophy (excessive growth) of the bone and soft tissue of the affected limb (which may lead to a difference in size between the affected and non-affected limb).
• Multiple arteriovenous fistulas (AVFs) due to abnormal connections between arteries and veins. These AVFs can be associated with life-threatening complications including abnormal bleeding and heart failure.

Some vascular abnormalities seen in Parkes Weber syndrome are similar to those that occur in a condition called capillary malformation-arteriovenous malformation syndrome (CM-AVM). CM-AVM and some cases of Parkes Weber syndrome have the same genetic cause.