Analgesic challenge was employed to identify fifty patients with aspirin idiosyncrasy. No serious reactions were observed using the method described. Patients with a history of the condition were highly sensitive and reacted to small doses of aspirin or to paracetamol. Patients with a positive challenge test, but previously unaware of aspirin idiosyncrasy, were less sensitive: they required larger challenge doses of aspirin and did not respond to paracetamol challenge. In the absence of an in vitro test, analgesic challenge is the only means of confirming the presence of aspirin idiosyncrasy.