Platelet may become activated following antigen challenge to participate then actively in the immune-inflammatory response. Moreover, some evidence proves that specific immunotherapy induces changes in the platelet function. The objective of this study was to determine circulating platelet activity during the early phase of allergen-specific immunotherapy (SIT) in patients with grass pollen-sensitive allergic rhinitis. Twelve grass-pollen allergic patients (seven men and five women) with intermittent allergic rhinitis were treated with specific subcutaneous allergoid preparation. SIT was received by six weekly injections, the vaccine dose increasing until the maintenance level was reached. Blood was sampled at four different time points: before and directly before SIT, 30 min and 24 h after the maximum dose injection of the vaccine. Plasma level of beta-thromboglobulin (beta-TG), marker of platelet activation in vivo was measured using ELISA method. Baseline beta-TG level did not differ significantly among the patients and healthy subjects. Moreover, no significant differences were observed in the degree of platelet activity between the different times of this study in the patients group. We failed to detect any significant changes in circulating platelet activity, the measure of plasma level of beta-TG, in patients with grass-pollen induced intermittent rhinitis during the course of the dose increase phase of grass pollen SIT. In particular, it seems that both early (after 30 min) and late (after 24 h) changes in plasma level of this marker do not occur following the maximum dose administration of the allergen vaccine during the early SIT phase.