Aspirin intolerance manifests itself as an acute urticaria-angioedema, bronchospasm, severe rhinitis, or shock occurring within three hours of aspirin ingestion. Aspirin intolerance occurs most commonly in patients with chronic urticaria (23 percent), in whom it is mostly manifested by the urticarial type of aspirin intolerance, and in asthmatic persons (4 to 19 percent), in whom it is mostly manifested by the bronchospastic type. The frequency of aspirin intolerance in patients with nasal polyps is about 23 percent, and consists of the bronchospastic type of intolerance. There is no definite evidence that aspirin intolerance is mediated by an immunologic reaction. In the bronchospastic type, an association between prostaglandins and the slow-reacting substance of anaphylaxis seems likely. The pathogenic mechanism for the urticarial type of aspirin intolerance is unknown. Under carefully controlled conditions, a transient desensitization to aspirin is possible. Various drugs, most of which are prostaglandin inhibitors, cross-react with aspirin in intolerant persons. They are, in decreasing order of frequency, as follows: indomethacin (100 percent), ibuprofen, mefenamic acid, phenylbutazone, sodium benzoate, tartrazine and acetaminophen (5 percent). Aspirin is an efficacious drug with increasing therapeutic use. In order to minimize its risk and increase its safety, physicians should familiarize themselves with those high-risk conditions where aspirin therapy should be avoided, or at least used cautiously.