Mastocytosis is a disorder of mast cell proliferation that may appear during infancy, childhood, or adulthood. We studied 67 consecutive patients (33 males, 34 females) with urticaria pigmentosa and assessed them fully to determine the presence of systemic involvement. Ages at onset of lesions ranged from birth to 11 years, with most developing in the first year of life. Pruritus was the primary symptom. Hematologic and serum chemistry profile, radiologic skeletal surveys, and bone marrow aspirations were performed. Slight anemia was present in three patients. Radiologic bone lesions were observed in eight. Bone marrow aspirates showed slight changes in six patients, with only an increased number of mast cells in an additional patient. The disease tended to resolve spontaneously. This prospective study emphasizes the benign nature of pediatric urticaria pigmentosa.