Mastocytosis can be associated with other clonal or non-clonal hematologic diseases as well as a variety of non-hematologic malignancies. A 75-year-old Caucasian male patient was referred to us with a 5-month history of neutrophilic leukocytosis and mild splenomegaly. He had developed a cutaneous melanoma sixteen years ago. According to the clinical and pathological features, a final diagnosis of systemic mastocytosis was established. The patient started treatment with interferon-α at a dose of 3 MIU/day, combined with low doses of prednisone. We observed a rapid disappearance of symptoms. Unfortunately, after 3 months a diagnosis of pancreatic adenocarcinoma was established. A review of the literature suggests that mastocytes could have a pivotal role in several malignancies. Different chemokines, mitogenic factors, chemical mediators of inflammation, and specific gene mutations could explain the association between mastocytosis and other hematologic and non-hematologic disorders.