Activated T cells and their cytokine products are involved in the pathogenesis of asthma. However, little is known about changes in circulating T cell subsets in allergic asthma during natural exposure to allergens. We examined whether natural allergen exposure of patients with atopic asthma is associated, in vivo, with changes of lymphocyte subtypes and activation markers in peripheral blood. Ten patients with atopic mild asthma sensitized only to grass pollen had peripheral venous blood lymphocyte analyses before and during the pollen season. No significant changes were observed. There was an inversion in the CD4/CD8 ratio in the peripheral blood both before (p < 0.05) and during (p < 0.01) seasonal exposure when compared to a group of healthy, age-matched control subjects. Evaluation of T cells expressing CD25 activation marker also demonstrated a significant reduction of CD4+25+ cells and a significant increase of CD+25+ cells compared to the controls. CD23+ cells (B lymphocytes with low affinity Fc IgE receptor) in the asthmatic group out of the pollen season correlated negatively with hyperreactivity to methacholine (p < 0.05). We conclude that in mild allergic asthmatic patients sensitized to grass pollen, blood lymphocyte subsets and their activation markers do not reflect seasonal exposure. Moreover, our findings show that these patients have higher proportions of CD8+ cells expressing higher levels of CD25 in their blood compared to normal subjects both before and during the pollen season.