Six out of eight patients with a history of aspirin-provoked urticaria/angioedema responded with adverse reactions, including urticaria and bronchospasm, to provoking doses of oral aspirin from 30-515 mg. The other two patients did not react to 1.2 g of aspirin on three occasions. Five of the six patients who had reacted became desensitized after their initial aspirin reaction, tolerating 650 mg on the second day. They then took 650 mg day−1 of aspirin for three weeks, during which time the ingestion of foods which had previously caused a variety of moderate or severe reactions caused no symptoms. The resting plasma PGF2α in ten ‘aspirin-sensitive’ urticaria patients (24.89 + 2-79 pg m−1) was significantly higher than the levels in ten normal subjects (6.75 + 1-1 pg ml−1) (P < 0.01). In the patient group the lowest levels of PGF2x were found in the two patients who subsequently did not experience a positive reaction after aspirin provocation. The PGF2α/PGE2 ratio in ‘aspirin-sensitive’ urticaria patients (1-83 + 0.026) was significantly higher than that in normal subjects (0-63 + 0.14) (P < 0.01).