Mastocytosis is characterized by an excessive number of apparently normal mast cells in the skin and, occasionally, in other organs. Characteristic skin lesions, called urticaria pigmentosa, are present in most patients, but clinical presentation can vary from a pruritic rash to unexplained collapse and sudden death. These lesions are typically tan to red-brown macules that appear on the trunk and spread symmetrically. Patients with mastocytosis often have a long history of chronic and acute symptoms that were unrecognized as mastocytosis. Skin lesions may or may not accompany systemic mastocytosis. Systemic disease may involve the gastrointestinal tract, the bone marrow or other organs. Even when the disease is considered as a possibility by the physician, the diagnosis can be difficult because of special technical requirements necessary for biopsy and because of the problems with biochemical testing. Drug therapy is initiated to stabilize mast cell membranes, to reduce the severity of the attacks and to block the action of inflammatory mediators. The mainstay of therapy is histamine H1 and H2 blockers and the avoidance of triggering factors.