Malignant systemic mastocytosis is a rare disorder, a subgroup of the mast cell neoplasms. Its clinical and histological diagnosis is often difficult, especially in patients without cutaneous involvement. We report an unsuspected case who underwent laparotomy complicated by life-threatening hypotension consistent with vasoactive mediator release from mast cells. The subsequent use of two chemotherapy regimens is detailed and both induced transient reduction in disease bulk.