We evaluated the effects of corticosteroid pretreatment on the intensity of adverse reactions to aspirin during carefully controlled aspirin challenges in 13 aspirin-sensitive asthmatics. Two studies were performed. First, using a double-blind crossover design, we administered to five patients 75 mg prednisone daily or placebo for two days preceding the challenge. No consistent protection against adverse reactions was achieved. In the second study, a 10-day pretreatment with 15 mg oral prednisolone and topical intrabronchial and intranasal beclomethasone offered total clinical protection against bronchospasm produced by threshold doses of aspirin in five of eight patients. In two others, the bronchospasm provoked was less severe, and only in one patient did its intensity remained unchanged. There was a significant reduction in fall of mean pulmonary function tests following the second aspirin challenge, which was performed after ten days of steroid treatment. Steroids given for ten days also prevented significantly nasal discharge and nasal blockade. When diagnostic challenge tests with aspirin are carried out in asthmatic patients on long-term corticosteroid therapy, there is an increased possibility of false negative results.