The nomenclature proposed in the October 2003 report of the Nomenclature Review Committee of the World Allergy Organization is an update of the European Academy of Allergology and Clinical Immunology Revised Nomenclature for Allergy Position Statement published in 2001. The nomenclature can be used independently of target organ or patient age group and is based on the mechanisms that initiate and mediate allergic reactions. It is assumed that as knowledge about basic causes and mechanisms improves, the nomenclature will need further review.