Malignant blue nevus arising in a giant congenital cellular blue nevus in an infant.

Giant congenital blue nevus (GCBN) is rare and usually occurs on the scalp. Malignant blue nevus (MBN) is also rare and has a poor prognosis. We report a case of MBN arising in a GCBN on the back. There have been three previous reports of MBN associated with GCBN on the trunk; our case had the earliest onset of MBN arising in a GCBN.