A lesion of nevus sebaceus usually is present at birth and typically on the scalp, where it appears as an alopecic, slightly yellowish macule. For years it remains flattish and grows at the same pace as the growth of the child who bears it. At puberty, however, the lesion tends to become mammillated and more yellow. As time goes on, the surface may become increasingly papillated and may even become verrucous.
Benign neoplasms develop commonly in fully-formed lesions of nevus sebaceus, chief among them trichoblastomas that are often markedly pigmented hemispherical papules. Much less often a malignant neoplasm arises in a nevus sebaceus, usually a trichoblastic (basal-cell) carcinoma. Trichoblastomas are at least 10 times more common than basal-cell carcinomas in lesions of nevus sebaceus. Once a lesion of nevus sebaceus has come into being, it persists for a lifetime.