Urticaria pigmentosa (UP) is a disorder of mast cell proliferation that occurs in cutaneous tissue. Most patients whose skin manifestations appear in infancy or childhood, experience a resolution of the disease by adolescence. In order to elucidate the relationship between mast cell character and UP prognosis, we used an immunohistochemical approach to examine the expression of stem cell factor (SCF) and c-Kit in the skin of patients with UP. The results revealed intercellular SCF expression throughout the dermis in improving cases. On the other hand, in cases with a tendency to worsen, dermal SCF was recognized only partially or not at all. Regardless of the clinical course, intracellular SCF immunoreactivity of the entire epidermis increased in cases of child onset UP. The c-Kit expression of mast cells in all UP patients showed no relation to clinical features. These findings suggest that SCF in the dermis promotes the differentiation of mast cells infiltrating in UP, and might be an attractive candidate to induce the remission of UP.