Urticaria pigmentosa is the most common manifestation of mastocytosis, with
the majority of cases undergoing spontaneous resolution, especially in chi
ldren. Several reports have documented hematologic malignancies developing
in patients with urticaria pigmentosa. We present a 4.5-year-old boy with u
rticaria pigmentosa who developed Wilms tumor. To our knowledge, coexisting
urticaria pigmentosa and Wilms tumor have not previously been described.