Reye's syndrome was a rare disease which appeared suddenly in the early 1950s and disappeared just as suddenly in the late 1980s. An association between Reye's syndrome and the ingestion of aspirin (acetylsalicylic acid) was claimed, although no proof of causation was ever established. The presence of salicylates in the blood or urine of Reye's syndrome patients has not been demonstrated, and no animal model of Reye's syndrome has been developed where aspirin causes the disease. It is clear from epidemiological data that the incidence of Reye's syndrome was decreasing well before warning labels were placed on aspirin products. Reye's syndrome disappeared from countries where aspirin was not used in children as well as from countries which continued to use aspirin in children. Reye's syndrome was probably either a viral mutation which spontaneously disappeared, or a conglomeration of metabolic disorders that had not been recognized or described at that time.