Summary. Systemic mastocytosis (SM) is characterized by proliferation of mast cells in various organs, which may release a wide variety of mediators, thereby explaining the broad clinical spectrum of disease manifestations. The potentially life-threatening systemic symptoms and tumoral proliferation are poorly controlled despite the use of several cytotoxic chemotherapies and/or symptomatic treatments. Twenty consecutive adult SM patients with histologically confirmed bone marrow (BM) involvement received interferon-α subcutaneously (1–5 million units/m2/d, with progressive dose intensification over the first month of treatment) and were evaluated after 6 months of therapy. Seven of them had previously received symptomatic treatments, including steroids, which were ineffective. Among the 13 patients treated for at least 6 months, seven partial and six minor responses, mainly concerning vascular congestion and skin lesions, were obtained, while BM infiltration remained unchanged in 12 patients. The significant reduction of mast-cell mediator levels after 6 months of treatment was not predictive of clinical remission. The rate of depression was unexpectedly high (seven patients; 35%). Two patients died soon after starting therapy (one myocardial infarction, one septic shock). Six months of interferon-α may relieve vascular congestion in adults with SM, probably by inhibiting mast-cell degranulation.