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In a 1993 survey, allergic rhinitis was identified as the most common allergic disease in Hong Kong, affecting 29.1% of schoolchildren. Recently (1995), the International Study of Asthma and Allergies in Childhood (ISAAC) also reported 44.5% current rhinitis among Hong Kong teenagers. Our objective was to study the aetiology of allergic rhinitis in Hong Kong using serological tests of allergen sensitization. In 57 allergic rhinitis patients and in the same number of age- and sex-matched controls the following were measured: serum total IgE, mixed aeroallergen IgE (Phadiatop™) and specific IgE versus house dust mite (HDM), cockroach, cat and dog dander, mould mixture (Penicillium, Cladosporium, Aspergillus and Alternaria species) and four local pollens (Bermuda grass, Timothy, ragweed and mugwort). Compared with controls, allergic rhinitis patients (26 males, 31 females; mean (± SD) age 25 ±11 years) had a significantly elevated serum total IgE concentration (mean ± SEM: 496 ± 88 vs 179 ± 38 kU/L) and an increased proportion of positive Phadiatop (95 vs 33%) and specific IgE tests versus HDM (90 vs 44%) and cockroach (42 vs 9%; Mann-Whitney U-test and χ2 tests all P < 0.005). There was no significant difference in sensitization to other allergens tested. House dust mite and cockroach are ubiquitous in Hong Kong with a warm, humid climate and crowded living conditions. Their identification as aetiological agents of allergic rhinitis should help in the development of environmental strategies for reducing the inhalant allergen load to prevent and control this prevalent and costly health problem in our community.