This is a birthmark with unknown cause. This usually occurs on the upper and middle third of one side of face, but rarely can also happen on both sides. The ears, inside of mouth, and the sclera of the eyes can also be affected. This is most commonly found in Asians and Blacks and mainly affects women. About 1 - 2% of the Japanese population has it but it is very uncommon in Caucasians.

If the eye is affected, arrange for regular eye examinations to detect glaucoma.

On the skin, the colour ranges from bluish gray to dark brown, with an edge blends gradually with the adjacent normal skin. The melanin and melanocytes are found deep in the dermis and laser treatment is the only hope for fast and effective clearance. The most effective laser so far is the Q-switched type with a wavelength of 532, 694, 755 or 1064nm. However many sessions, frequently up to around 10, are required to clear it enough not to be noticeable. Recurrence is common after a few years.