Background: Aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) can precipitate adverse reactions in two apparently different clinical conditions: bronchial asthma and chronic idiopathic urticaria (CIU). Recent evidence indicates that the reactions are triggered by the drugs that inhibit cyclooxygenase-1 but not cyclooxygenase-2. Objective: To assess whether patients with CIU and aspirin sensitivity share common eicosanoid alterations with patients who have aspirin-sensitive asthma. Methods: Seventy-four patients with CIU and a history of sensitivity to aspirin and NSAIDs underwent placebocontrolled oral aspirin challenge tests. Concentrations of urinary leukotriene E4 (uLTE4) were measured by ELISA and plasma stable prostaglandin D2 metabolite, 9a,11b prostaglandin F2 by GC/MS. All measurements were carried out at baseline and after aspirin dosing. Patients were genotyped for the leukotriene C4 synthase (LTC4S) promoter single nucleotide polymorphism. Results: In 30 of 74 patients, the aspirin challenge was positive, resulting in urticaria/angioedema. In these 30 patients, baseline uLTE4 levels were higher than in nonresponders and the healthy control subjects and increased further (significantly) after the onset of clinical reaction. No such increase occurred in subjects with negative aspirin challenge. Baseline uLTE4 levels correlated with severity of skin reactions. Plasma 9a,11b prostaglandin F2 levels rose significantly in both aspirin responders and nonresponders, although in the latter group the increase occurred later than in the former. In patients who reacted to aspirin, frequency of e444C allele of LTC4S was significantly higher than in patients who did not react. Conclusions: CIU with aspirin sensitivity is characterized by the eicosanoid alterations, which are similar to those present in aspirin-induced asthma.