Heparin possesses anti-inflammatory properties, which appeared to be dependent on the dose, timing, and the route of administration in animal studies. In asthma, a single dose of inhaled heparin only slightly reduced the early asthmatic response (EAR) but failed to protect against the late asthmatic response (LAR) to inhaled allergen. We studied the effect of multiple doses of inhaled heparin on the EAR and LAR to inhaled house-dust mite extract in eight stable asthmatics in a two-period, randomized, double-blind, crossover study. During both study periods, a standardized allergen challenge was performed and PC20 histamine was measured 24 h before and 24 h postallergen. Five doses of unfractionated heparin sodium (1,000 U/kg/dose) or placebo were inhaled 90 and 30 min preallergen, and 2, 4, and 6 h postallergen. Airway response was measured by FEV1, and the EAR (0-3 h) and LAR (3-10 h) were expressed as corresponding areas under the time-response curves (AUC). The acute effects of heparin and placebo on baseline FEV1 were not different (p > 0.07). Although not reaching significance, heparin attenuated the EAR by an average of 40% (mean AUC(0-3) +/- SEM: 29.5 +/- 6.0 [placebo] and 17.8 +/- 5.5% fall x h [heparin]; p = 0.08), while it significantly reduced the LAR by an average of 36% (AUC(3-10) +/- SEM: 169.3 +/- 20.0 [placebo] and 109.1 +/- 23.6% fall x h [heparin]; p = 0.005). We conclude that inhaled heparin reduces the LAR to allergen in asthmatic subjects, which may be due to its anti-inflammatory activity. Our finding suggests that heparin may have potential as anti-asthma therapy.