In addition to the well-recognized allergic responses of individuals to high mol wt substances, such as pollens, molds, and animal dander, susceptible asthmatics may also experience adverse reactions to low mol wt substances such as sulfites, ASA, and NSAIDs. The diagnosis of sulfite and aspirin sensitivity can only be made by appropriately conducted provocative challenge. Every precaution should be taken to assure the safety of the patients, since life-threatening reactions can occur. A better understanding of the mechanism or mechanisms involved in the adverse reactions to these substances will not only provide information to better diagnose the reaction, but also improve our understanding of the treatment of asthma.