Objective: To report the clinical responses and mediator-release profiles of an aspirin-sensitive man with systemic mast cell disease during aspirin desensitization. Material and Methods: We quantified the release of six mediators during aspirin desensitization. Results: Although aspirin was administered cautiously with an initial dose of 20 mg, successful aspirin desensitization necessitated complete monitoring and resuscitation capabilities of a medical intensive-care unit for 4.5 days because of frequent, severe anaphylactoid responses. To our knowledge, this is the first report of a pronounced increase in plasma levels of the vasodilator peptide calcitonin gene-related peptide during episodes of aspirin-induced hypotension, Increases in plasma levels of calcitonin and serum levels of tryptase paralleled those of calcitonin gene-related peptide, but plasma levels of calcitonin remained increased for up to 18 hours, Urinary excretion of histamine and 1-methyl-4-imidazoleacetic acid also showed precipitous, although delayed, increases. Excretion of the prostaglandin D-2 metabolite 11 beta-prostaglandin F-2 alpha a followed a bimodal pattern during aspirin desensitization; after severe hypotensive responses, the maximal value was more than 490,000 pg/mL, but the level decreased to less than 100 pg/mL after therapeutic serum levels of salicylate were attained, Conclusion: These data suggest that the hypotensive responses to aspirin in some patients with systemic mast cell disease may result from the combined effects of several mediators.