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A port-wine stain or naevus flammeus is a vascular birthmark consisting of superficial and deep dilated capillaries in the skin which produce a reddish to purplish discoloration of the skin. They are so called for their colour, resembling that of port wine. It is part of the family of disorders known as vascular malformations.

Port-wine stains are present at birth and persist throughout life. The area of skin affected grows in proportion to general growth. Port-wine stains occur most often on the face but can appear anywhere on the body. Early stains are usually flat and pink in appearance. As the child matures, the color may deepen to a dark red or purplish color. In adulthood, thickening of the lesion or the development of small lumps may occur.

PWS may be one of a group of symptoms and signs, in which case it is considered to be part of a syndrome such as Sturge-Weber syndrome or Klippel-Trenaunay-Weber syndrome. All birthmarks should be evaluated by the health care provider during a routine examination.