We report a case of aggressive systemic mastocytosis in a 3-year-old girl, who had undergone treatment for ovarian germ cell tumor during the previous 8 months. On diagnosis of systemic mastocytosis, she was treated with interferon-alpha and steroids. She showed tolerable side effects of interferon-alpha infusion, but died of multiple organ failure after 2 months of treatment. Point mutations of the C-KIT gene, previously implicated in the genesis of mastocytosis, were discovered not only in the bone marrow and the peripheral blood of the patient, but also in the tissue of the previously diagnosed germ cell tumor as well.