BACKGROUND Previously we reported that patients sensitized to pollen-allergens who had seasonal allergic rhinitis and seasonal asthma may show increased plasma levels of platelet activation markers during grass pollen season. OBJECTIVE To find out whether the pattern of platelet activity measured by plasma PF-4 level in the same group of patients changed off-season compared with the pollen season as well as in comparison with healthy control subjects. METHODS Off-pollen season, plasma PF-4 level was determined by enzyme-linked immunosorbent assay (ELISA) in 16 grass pollen allergic patients who had seasonal allergic rhinitis and seasonal asthmatic symptoms (none of them had any history of allergic diseases outside the season) and in 29 healthy nonatopic subjects. RESULTS Plasma PF-4 level in the patients off-pollen season was significantly lower as compared with the season and did not differ significantly as compared to the healthy subjects. CONCLUSIONS This observation taken along with our previous results indicates that patients with seasonal allergic rhinitis and asthma symptoms may have an increased circulating platelet activation, yet this phenomenon disappears in the asymptomatic period of the disease. This might indicate that platelet activation within the systemic circulation is an important factor in the development of seasonal allergic airway inflammation.