Patients with xeroderma pigmentosum (XP) have more than a 1000-fold increased risk of cutaneous melanoma. To determine if the XP DNA repair defect is present in cutaneous pigmentary cells, nevus cells and melanocytes from four large, pigmented nevi were cultured from a 12-year-old girl with XP. Cultured melanocytes showed dendritic morphologic features, contained mature melanosomes, and reacted with monoclonal antibody to tyrosinase. Nevus cells were spindle shaped and expressed nevus cell-associated antigens. Melanocytes, nevus cells, and dermal fibroblasts from the patient with XP all had a similar reduction in DNA repair: unscheduled DNA synthesis was 30% to 50% of that in normal fibroblasts following a 30 J/m2 ultraviolet dose. After a 6 J/m2 ultraviolet dose, the proliferative ability of XP nevus cells and fibroblasts was reduced to 10% of that of normal fibroblasts. This study indicates that cultured melanocytes and nevus cells express the characteristic XP DNA repair defect.