Conflicting results have been reported on the effect of non-steroidal antiinflammatory drugs on allergen-induced asthmatic responses. The aim of this study was to investigate the effect of inhaled lysine acetylsalicylate (LASA) on the early and late allergen-induced responses. We studied 16 patients with mild, stable asthma who had an early asthmatic response and 10 patients with a dual (early and late) response. Each patient underwent two challenges with a single dose of allergen assessed in a preliminary test, after inhalation of either 720 mg of LASA in 4 ml of saline solution or placebo, according to a randomized, double-blind protocol. Allergen-induced hyperreactivity to methacholine was measured in six patients from each of the early and the dual response groups 2 hours and 24 hours after the challenge, respectively. In the patients with early response, the maximum fall in FEV1 after challenge was 24% +/- 1% after inhalation of placebo and 14% +/- 2% after inhalation of LASA (p < 0.005). No protection was observed in four patients who received the drug orally instead of by inhalation. In the patients with a dual response, the maximum FEV1 decrease during the early response was 27% +/- 2% after placebo and 21% +/- 2% after LASA (p < 0.025). During the late response (between 3 and 8 hours), the maximum decrease in FEV1 was 28% +/- 4% after placebo and 16% +/- 4% after LASA (p < 0.005). In both groups allergen challenge caused a significant reduction in methacholine PD20 after treatment with placebo but not with LASA. Without allergen challenge, LASA had no effect on methacoline reactivity. We conclude that inhaled LASA significantly reduces both the early and the late asthmatic response to allergen challenge and that it prevents the allergen-induced airway hyperresponsiveness that follows these responses.