Summary Azathioprine has been used as an immunosuppressant for over 20 years in cancer chemotherapy, organ transplantation and diseases with confirmed or suspected immune mechanisms. A hypersensitivity reaction manifesting as fever, rash, myalgia and a neutrophil leucocytosis occurring about 2 weeks after exposure is well documented and has been confirmed by challenge testing. Hypotensive reactions are less common but potentially fatal; a case is reported where repeat exposure resulted in profound circulatory collapse responding only to intervention with inotropic agents.