We examined the relationships between exposure to different indoor allergens and demographic factors in 515 adult Taiwanese patients with allergic diseases such as asthma and rhinitis. Patients who had positive screening results on an immunoglobulin E (IgE) test were recruited from the outpatient clinics of Chang Gung Memorial Hospital. Patients were divided into the following three groups: asthma (n = 332), combined asthma and allergic rhinitis (n = 165), and allergic rhinitis (n = 18). Serum samples were analyzed for IgE by enzyme-linked immunosorbent assay. Significantly elevated titer of dust mite-specific IgE were found in all three groups, with markedly higher levels in the combined asthma and allergic rhinitis group (p < 0.05). Allergen scores were significantly related to the demographic variables of older age (> or = 65 years), female gender, and spring season, with significantly lower scores (p < 0.05). No significant difference in allergen scores was found among cigarette smoking subjects and subjects from residential areas. Multiple linear regression analysis revealed significant allergic contributors to be, in order of importance, age, gender, and atopic disease category. We conclude that mite allergen exposure is the most significant factor associated with asthma and/or allergic rhinitis in Taiwan. Younger subjects and males had a higher rate of asthma and/or allergic rhinitis.