A comatose patient arrived at the emergency room with apnea and in shock after a grand mal seizure. A maculopapular rash, not initially observed, led to a diagnosis of cutaneous mastocytosis. Degranulation of the mast cells with massive histamine release had resulted in cardiovascular collapse. The authors follow a detailed report of this case history with a discussion of the diagnosis and treatment of the acute systemic stage of this potentially life-threatening illness.