The process underlying anaphylaxis involves an uncontrolled elevation in blood levels of mediators, including histamine. Usually, these abnormal levels are attributed to the degranulation of basophils and mast cells. Few reports have assessed the contribution of defects in histamine pharmacodynamics to allergic responses. In this report we describe a patient with recurrent anaphylaxis who was initially suspected to have enhanced histamine intolerance. We evaluated urine and blood samples collected from this patient and from control individuals using an ELISA test. Our data clearly show constitutive hyperhistaminaemia and a markedly impaired urinary histamine clearance ratio in the index patient. It is suggested that this defect facilitates anaphylaxis.