This study aimed to measure the prevalence of allergic rhinitis among European adults and the proportion of undiagnosed cases in Belgium, France, Germany, Italy, Spain, and the UK. The research was conducted through two steps: a telephone interview screening and a clinical diagnosis confirmation. In the first step, 9,646 telephone interviews were conducted, with 19% of subjects reporting self-awareness of allergic rhinitis and 15% reporting a physician-based diagnosis. In the second step, 725 clinical assessments were performed, confirming allergic rhinitis in 411 subjects. Among these, 45% had not been previously diagnosed by a physician. The overall prevalence of clinically confirmable allergic rhinitis ranged from 17% in Italy to 25% in Belgium, with an overall estimate of 25%. The study confirmed the high prevalence of allergic rhinitis in western Europe and the significant underdiagnosis of the condition.This study aimed to measure the prevalence of allergic rhinitis among European adults and the proportion of undiagnosed cases in Belgium, France, Germany, Italy, Spain, and the UK. The research was conducted through two steps: a telephone interview screening and a clinical diagnosis confirmation. In the first step, 9,646 telephone interviews were conducted, with 19% of subjects reporting self-awareness of allergic rhinitis and 15% reporting a physician-based diagnosis. In the second step, 725 clinical assessments were performed, confirming allergic rhinitis in 411 subjects. Among these, 45% had not been previously diagnosed by a physician. The overall prevalence of clinically confirmable allergic rhinitis ranged from 17% in Italy to 25% in Belgium, with an overall estimate of 25%. The study confirmed the high prevalence of allergic rhinitis in western Europe and the significant underdiagnosis of the condition.