Systemic mastocytosis is a chronic disease with proliferation of mastocytes in organs separate from skin. We report the case of a systemic mastocytosis revealed by a lymphocytic ascites in the absence of cutaneous involvement. The diagnosis was established by the ultrastructural examination of the bone marrow. Identification of few mastocytes in ascites suggested a mast cell infiltration of the peritoneum. A treatment by alpha-2b interferon was unsuccessful, and death rapidly occurred.