RESUMO
Nevus spilus is the term usually given to a pigmented skin lesion, congenital or acquired, that may occur anywhere on the body, consisting of a large light tan patch with numerous superimposed darker scattered maculae or papulae that are flat or slightly raised. For a long time, nevus spilus was believed to be a benign lesion. However, in 1957 Perkinson reported a melanoma appearing on nevus spilus for the first time. Since then other reports about melanomas developing on nevus spilus have been published, sometimes with a fatal outcome. We describe the case of an 80-year-old male patient with a congenital nevus just above his left knee. The lesion had remained unchanged over time, but some months before his checkup the patient noticed a darker area in the lesion that had continued to enlarge. The lesion was removed and histological examination revealed an in situ malignant melanoma. Although nevus spilus is not normally considered a precursor of melanoma, the potentiality of malignant transformation requires regular monitoring, and careful checkups are recommended and justified.