The kinetics of the immunological mechanisms during allergen-specific immunotherapy (IT) has not been thoroughly evaluated. In this investigation we study the changes in T-cell responses during the early phase of IT. Ten patients (IT group) with birch-pollen allergy were treated with conventional IT. Blood samples were collected at regular intervals for specific immunoglobulin (Ig)E measurements and preparation of peripheral blood mononuclear cells (PBMC). Seven allergic control patients (AC group) were included during the subsequent birch-pollen season. The PBMC were stimulated with birch-pollen extract or tetanus toxoid (TT) and mitogens. After a short decrease, probably owing to seasonal variation, the birch-pollen-specific proliferation and the interleukin (IL)-4, IL-5, and IL-10 production significantly increased when reaching the maintenance dose and during the subsequent pollen season. The increase in IL-4 correlated with a temporary increased serum level of birch-pollen-specific IgE. Interestingly, also the TT-specific response was affected by IT, resulting in weaker, but in time similar, changes in proliferation and cytokine production as in the birch-pollen-specific response. We speculate that the early phase of IT might lead to systemic changes in the capacity of Th2-like cytokine production, and that the early increase in allergen-specific IgE is a consequence of enhanced IL-4 production.