A capillary malformation (commonly referred to as a port-wine stain), is a flat, sharply defined vascular stain of the skin. It may cover a large surface area or it may be scattered and appear as smaller spots of color. It is usually located on one side of the body without crossing the midline. It can be found anywhere on the body, most commonly the head / neck region.

Recently, a genetic change (mutation) was found in the affected skin from capillary malformations but was not present in nearby normal skin. This pattern tells us that capillary malformations occur in early development when tiny blood vessels of the skin form abnormally as a result of faulty instructions. The occurrence of capillary malformations is unrelated to drugs or medications that may have been taken during pregnancy or to environmental exposures that may have occurred at that time.

Another capillary vascular lesion, called nevus simplex, is mostly seen in newborns and is often confused with capillary malformations. When these birthmarks appear on the forehead, eyelids, nose or upper lip, they are commonly called "angel kisses." When seen on the back of the neck, they are commonly called "stork bites." These lesions usually fade by 1 year of age and do not require treatment.