BACKGROUND Animal models of allergic asthma indicate that intravascular platelet activation is necessary for the development of allergen-induced chronic airway inflammation. OBJECTIVE To evaluate whether the development of a late asthmatic response (LAR) in allergic asthma patients challenged with a relevant allergen is consequent to platelet activation. METHODS Thirty-three house dust mite sensitive asthmatic patients were challenged intrabronchially with Dermatophagoides pteronyssinus (Dp) extract. Twelve non-atopic healthy subjects (HC) were used as controls. Platelet count and plasma levels of beta-thromboglobulin (beta-TG), platelet factor-4 (PF-4) and soluble P-selectin (sP-selectin) were assessed before the challenge (T(0)) and 30 min (T(EAR)), 6 h (T(LAR)) and 24 h (T(24)) after the challenge. RESULTS Eleven patients responded to allergen challenge with an isolated early asthmatic response (single responders, SR). In 22 patients dual asthmatic response was demonstrated (dual responders, DR). At T(0) neither the platelet count nor the mean plasma level of beta-TG in DR or SR were different from HC, the mean plasma level of PF-4 in SR was significantly greater than in HC (P=0.01) or DR (P=0.001), the mean plasma level of sP-selectin was significantly greater in DR than in HC (0.0002) but not statistically different from SR (P=0.055). A significant decrease in the platelet count and increase in the plasma level of all the studied markers was seen at T(EAR), which was followed by a gradual return to the baseline values in the SR. Elevated plasma levels of platelet activation markers and decreased platelet count were seen in the DR even at T(24). Strong correlation was found between the increase in plasma concentration of beta-TG at T(EAR) and the maximum fall in forced expiratory volume in 1 s at T(LAR) (r=-0.57; P=0.0006). CONCLUSIONS In allergic asthma patients development of prolonged airway inflammation after allergen challenge is associated with intravascular platelet activation.