Giant congenital melanocytic nevus with underlying hypoplasia of the subcutaneous fat.

A 17-week-old boy with a giant congenital melanocytic nevus (GCMN) of the left lower extremity was noted to have a reduction in circumference of the left lower extremity relative to the contralateral side. The skin overlying the GCMN was persistently warm when compared with the surrounding and contralateral skin. Comparative plain radiography, ultrasonography, and magnetic resonance imaging showed fat hypoplasia of the left lower extremity, with bone and muscle appearing unaffected.