Background: The recognition of allergenic peptides by T cells through their T-cell receptor (TCR) represents a crucial step in the initiation of an allergen-specific immune response. In parallel to the superantigen-driven restricted expansion of Vβ subsets in autoimmune and infectious diseases, reports in animals and human subjects have shown a similar capacity of classical antigens. Objective: The study was performed to analyze the V⍺/Vβ expression in house dust mite (HDM) allergy. Methods: The TCR repertoire of 15 subjects with HDM allergy, 22 atopic subjects without HDM allergy, and 19 nonatopic individuals, members of 2 extended and 4 nuclear families, was determined. By using flow cytometry, the expression of 22 Vβ and 3 V⍺ elements was analyzed in vivo and after in vitro allergen stimulation. Results: In comparison with nonatopic and atopic individuals without HDM allergy, freshly isolated PBMCs of individuals with HDM allergy showed a significantly higher frequency of Vβ18⁺ and V⍺2.3⁺ T cells. Although members of all 3 groups had a similar lymphocyte proliferation response after in vitro stimulation with Der p 1 or Der p 1 peptide_101-131, a significant expansion of Vβ18⁺ and V⍺2.3⁺ T cells in vitro occurred only in individuals with HDM allergy. Moreover, the degree of expansion correlated with the levels of allergen-specific IgE antibodies. No expansion of Vβ18⁺ and V⍺2.3⁺ was observed after mitogen stimulation with PHA, indicating allergen specificity of the response. Conclusion: Our results strongly suggest restricted TCR V⍺/Vβ gene use in HDM allergy and might be a step toward TCR-based immunotherapy. (J Allergy Clin Immunol 2002;109:517-23.)