A port-wine stain is a mark that is usually present at birth. It is made of enlarged blood vessels. This makes it appear as a reddish-purple patch of skin.

Port-wine stains are generally harmless. They may cause emotional and social problems due to their visibility.

Port-wine stains are caused by a problem with the small blood vessels in the skin. Blood vessels can normally open and close to meet the needs of the skin. In port-wine stains, the blood vessels stay wide open. Blood fills the vessels causing the purple color and raised skin. It is not clear what causes the problems with the blood vessels.