A port-wine stain is a type of birthmark that appears as a flat, reddish-purple mark on the skin. It is caused by an overabundance of blood vessels in a specific area and can vary in size and shape. These marks are usually present at birth and can occur anywhere on the body, but they are most commonly found on the face and neck. Port-wine stains are classified as a type of vascular malformation. They do not fade over time and may darken or thicken as a person ages. While they are generally harmless, some individuals may seek treatment for cosmetic reasons or if the stain is located in a sensitive area, such as near the eyes.