The skin of six cases of incontinentia pigmenti was studied electron microscopically. The patients ranged from 9 days to 34 years of age and included one male. Ultrastructural alterations observed confirmed the clinical staging of the disease. Early in the illness both deposition of melanin into the dermis by epidermal melanocytes and an abnormal population of melanocytes high in the epidermis were noted. Also deposition of melanin in Schwann cells was seen. With the passage of time there was first dysfunction and finally atrophy of melanocytes and dispersal of the dermal deposits of melanin.