To measure the prevalence of allergic rhinitis among European adults and the proportion of undiagnosed subjects, a two-step, cross-sectional, population-based survey in Belgium, France, Germany, Italy, Spain, and the UK was undertaken. Step one of the study involved screening for allergic rhinitis by telephone interview, based on history of symptoms and/or self-awareness of the condition. Step two undertook confirmation of allergic rhinitis in a subset of the subjects screened positive; this was performed by a clinical diagnosis conducted in three to five clinical centres per country, including specific immunoglobulin E tests and a disease-specific questionnaire. A total of 9,646 telephone interviews were conducted between February and April 2001. Self-awareness of allergic rhinitis was reported by 19% of the subjects. Physician-based diagnosis of allergic rhinitis was reported by 13% of the subjects. In step two, 725 clinical assessments were conducted between May and August 2001. A total of 411 of patients, who underwent step two, had investigator-confirmed allergic rhinitis. Among patients with investigator-confirmed allergic rhinitis, 45% had not reported a previous diagnosis by a physician. Prevalence of subjects with clinically confirmable allergic rhinitis estimated by combining step one and step two data ranged from 17% in Italy to 29% in Belgium with an overall value of 23%. This large-scale study confirms that allergic rhinitis has a high prevalence in western Europe and is frequently undiagnosed.