A 70-year-old man presented with a birthmark on the right upper arm that had been present for as long as he could remember (Figure). His physician had referred him for a dermatology consultation because the lesion had become larger and darker over the past year. What are the next steps in the evaluation and management of this patient? The authors discuss the diagnosis and management of a changing congenital melanocytic nevus. The epidemiology and risk of transformation into melanoma are reviewed. Techniques of biopsy and indications for follow-up are discussed. A brief review of giant congenital melanocytic nevi in the newborn is also included.