Cutaneous epidermoid cysts are very common lesions, but neoplastic transfor
mation of their epithelium is extremely rare. We describe a patient with ba
sal cell carcinoma arising in the wall of an epidermold cyst present on the
back for 20 years. We emphasize the advisability of a close histologic exa
mination of all cutaneous cysts that are seemingly benign in order to avoid
incorrect diagnosis.