The clinical history of more than 1000 aspirin-sensitive patients who have been under observation from 1954 until 1965 is analyzed. Administration of various salicylate esters confirms the unique effect of the acetyl ester; several chemically unrelated analgesics, produce reactions as severe as the reactions caused by aspirin. Intolerance to aspirin may be caused by the pharmacological action of the drug on peripheral chemoreceptors.