Epidermal nevi (EN) are benign lesions presenting at birth or in childhood. Based on the occurrence of fibroblast growth factor receptor 3 (FGFR3) mutations in seborrheic keratosis and urothelial carcinomas (UC), and the identification of two young patients with EN and UC, we hypothesized that mutations might occur in EN. The R248C mutation was found in 6/23 (26.1%) EN but it was absent from unaffected skin. In two patients with EN and UC, both lesions were FGFR3 wild type. Our findings indicate that: (1) FGFR3 mutations occur in mosaicism and can cause EN and (2) other genes are involved in EN. © 2007 The Society for Investigative Dermatology.