Mastocytosis comprises a heterogeneous group of hematological disorders which are morphologically defined by proliferation and accumulation of tissue mast cells in one or more organs. Clinical manifestations of mastocytosis range from disseminated maculopapular skin lesions (= urticaria pigmentosa [UP]) that may spontaneously regress to highly aggressive neoplasms like mast cell leukemia or mast cell sarcoma. Recently, it could be shown that systemic mastocytosis (SM) is a clonal disorder often exhibiting mutations of c-kit, a protooncogene encoding the tyrosine kinase receptor for stem cell factor (SCF). Mutations of c-kit are considered to play a key role in the pathogenesis of mastocytosis. Therefore, we investigated the unique case of a 36 year-old male patient with indolent systemic mastocytosis (ISM) evolving from UP (cutaneous mastocytosis) by means of histology, immunophenotyping and molecular biology. At the time of initial diagnosis the bone marrow showed only a mild diffuse increase in mast cells but compact infiltrates were missing. The serum tryptase levels were normal. Five years later, however, the bone marrow histology displayed patchycompact mast cell infiltrates, which now allowed to establish the diagnosis of an ISM. The serum tryptase levels at this time were markedly elevated. At both time points, mast cells were analyzed by immunohistochemistry using anti-tryptase antibody AA1, by flow cytometry using antibodies against CD2 and CD25, and nested polymerase chain reaction (PCR) on laser-microdissected, single pooled mast cells. Immunohistochemistry revealed strong tryptase-positivity of mast cells in both cutaneous and bone marrow infiltrates. Flow cytometry yielded an aberrant expression of CD2 and CD25 on bone marrow mast cells. However, repeated thorough PCR analysis failed to unveil c-kit mutation in atypical mast cells of skin and bone marrow samples of both dates. These findings clearly show that ISM can evolve from UP. Moreover, our study provides further evidence that the c-kit mutation Asp-816-Val is not invariably present in ISM.