The skin of 18 cases of xeroderma pigmentosum was examined electronmicroscopically. In all cases, the melanocytes were abnormal. They showed extensive melanosome polymorphism, the formation of large autophagic vacuoles; in eight cases, giant pigment granules of a unique character were present. There was passage of premelanosomes and giant pigment granules to keratinocytes. In a depigmented lesion, Langerhans’ cells were seen to be situated in the position normally occupied by melanocytes. Langerhans’ cells were also seen bearing large quantities of melanin.