Patients with pollen allergy may frequently present an additional food-related allergy (Oral Allergic Syndrome, OAS), as consequence of cross-reactivity between pollen allergens (mainly birch, hazelnut, alder, mugwort) and vegetable allergens. The aim of this study was to evaluate the effect on Bet v 1-induced T cell proliferation exerted by the presence of OAS in birch patients. Fourteen allergic patients were evaluated (6 males, mean age 35.8 years). All of them were monosensitized to birch and suffered from allergic rhinitis: 4 had also OAS to apple. Proliferation of peripheral mononuclear cells was evaluated using Bet v 1 and non-specific stimuli. OAS had higher proliferation than non-OAS patients. In addition, there were significant relationships between immunological and clinical parameters in OAS patients. This study evidences that OAS characterizes a more severe form of birch allergy: as OAS patients had higher SI, circulating eosinophils, and IgE levels. Thus, this study confirms the previous report and underlines the relevance of measuring recombinant birch allergen as higher values may suggest a reliable prediction of OAS.