Purpose: Pollinosis is one of the major allergic diseases caused by airborne pollens. Alder and birch pollens are the major sensitizing tree pollens in this country. The immunoglobulin E (IgE) reactivity to each pollen allergen is known to be variable according to the region. We determined the major IgE binding components of these tree pollens in sera of adult patients with allergic rhinitis. Methods: Allergic rhinitis patients, of whom specific IgE level to birch and/or alder pollens (>10 kU/L by ImmunoCAP) were included. The protein bands of two pollen extracts were determined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, and their IgE-binding components were identified by IgE immunoblot analysis. The binding specificity and cross-reactivity between two pollens were evaluated by IgE enzyme linked immunosorbent assay (ELISA) inhibition test. Results: Six IgE binding components were found in birch pollens in which two (14 kDa and 17 kDa) were major components. Two IgE binding components were found in alder pollens in which the 17 kDa was a major component. The IgE binding component to the major allergen component of 17 kDa was observed in 90.3% of the study subjects sensitive to alder pollens and 72.7% of them sensitive to birch pollens. The ELISA inhibition tests showed significant inhibitions with additions of birch/alder pollen extracts. Conclusion: We identified two major IgE binding components (17 kDa and 14 kDa) from birch pollens and one component (17 kDa) from alder pollens. Significant cross reactivity was noted between these two pollens. (Allergy Asthma Respir Dis 2013;1:216-220)