We report on a patient with systemic mastocytosis with an activating point mutation of the c-kit gene. This mutation was identical to the c-kit mutation recently described by other groups. Additionally, we found that in this patient the mutation was also present in myeloid and erythroid lineages, indicating a multilineage involvement and suggesting a clonal origin of the disease similar to that described in other myeloproliferative disorders. The erythroid involvement was further demonstrated by the presence of erythropoietin-"independent" erythroid progenitor cells.