Congenital nevus (by definition) is present at birth. They are usually larger than acquired nevi and may reach impressive proportions (greater than 20 cm).

Histologically, congenital nevi are characterized by an infiltration of melanocytes in the reticular dermis. Melanocytes may be present in the deep dermis and subcutis as nodules or single cells. They are commonly seen in in a perivascular and interstitial distribution, as well as involving adnexal structures, smooth muscle (e.g. arrector pili), nerve trunks and even walls of blood vessels. Cellular and proliferative nodules with mitotic activity may be common in large nevi. The melanocytes may be spindled or fusiform, lending a sarcmatoid appearance (Rapini, Busam).

Several features of congenital nevi may raise concern fo a melanoma including pagetoid spread melanocytes in the epdiermis and mitotically active proliferative nodules. To distinguish between the two entities, look for cytologic atypia and lack of maturation in a melanoma nodule. Clinical context will be critical.

Congenital nevi are either apparently at birth or within the first two years of life "congenital nevus tardive". There does not appear to be a race or gender predilection (Schwartz).

May occur in the context of neurocutaneous melanosis, a rare congenital syndrome of large, multiple melanocytic nevi in association with benign or malignant proliferation of melanocytes in the leptomeninges. Neurologic symptoms occur within the first two years of life due to increased intracranial pressure. Approximately half the patients develop CNS melanoma and the prognosis of symptomatic patients is very poor (Makkar).

Large congenital nevi may be a cosmetic problem, but also carry a risk of malignant transformation to melanoma. The risk of developing melanoma has been reported to be as high as 5-7% by age 60 years in giant congenital nevi (Schwartz).