Mastocytosis is a rare disease characterized by pathologically increased mast cells in various tissues. Cutaneous mastocytosis, the most common manifestation of the mastocytosis, is confined to the skin without involvement of the other organs. Cutaneous mastocytosis is mostly found in children. It usually presents as symmetrically distributed red-brown macules or papules. Wheals and erythema may often develop with pruritus if mechanical irritations such as stroking (Darier's sign), heat or sudden change of temperature are given to the skin lesions. Clinical symptoms include flushing, urticaria, angioedema, anaphylaxis, peptic ulcer disease, abdominal pain, diarrhea and bone pain caused by the release of mast cell mediators. We report a case of adult-onset cutaneous mastocytosis that presented as generalized urticaria developed after hot bath.