Urticaria pigmentosa is one of a spectrum of diseases of the mast cell, termed mastocytosis. This is an uncommon disorder in which mast cells are overproduced and accumulate in various organs. In urticaria pigmentosa, mast cells accumulate in the skin, causing a rash. In this article, the diagnosis and management of urticaria pigmentosa will be reviewed by examining a case occurring in a 5-month-old boy. There is also a brief review of systemic mastocytosis and other mastocytosis variants.