Port-wine stains occur most often on the face but can appear anywhere on the body, particularly on the neck, upper trunk, arms and legs.

[2] Nevus flammeus may be divided as follows:[6] Port-wine stains were shown to be caused by a somatic activating c.548G→A mutation in the GNAQ gene.

Depending on the location of the birthmark and other associated symptoms, a physician may choose to order a measurement of intraocular pressure or X-ray of the skull.

An MRI of the brain may be performed (under anesthesia) on infants who have a port-wine stain in the head area in order to check for signs of Sturge–Weber syndrome.

If swelling occurs in the port-wine stain, it may cause vision problems, glaucoma, or blindness.

Lasers and other light sources may therefore be able to reduce the redness of port-wine stains, although there is not enough evidence to recommend one form over another.

[2] If the port-wine stain is on the face or other highly visible part of the body, its presence can also cause emotional and social problems for the affected person.