Asthma in children and young adults is strongly associated with immediate hypersensitivity to indoor allergens, notably those derived from the house dust mite. In addition, outdoor air pollution is considered to aggravate existing asthma. We investigated the prevalence of asthma and the pattern of allergen sensitization in a mite-free environment with low levels of outdoor air pollution. A total of 567 children aged between 12 and 14 attending Los Alamos Middle School, NM (altitude 7,200 feet) were screened using a respiratory questionnaire; 120 children (53 control children) underwent allergen skin testing and serum IgE measurement, and their bronchial reactivity to histamine was measured. Dust was collected from 111 homes and the level of indoor mite and cat allergen measured. The prevalence of respiratory symptoms was high (13%), and from the detailed testing it was estimated that 6.3% of the children had asthma (defined as symptomatic bronchial reactivity). Children with asthma had elevated IgE, 367 (179 to 755) versus 38 (23 to 61), and predominant sensitization to cat, 68 versus 20% (p < 0.001). A high number of households (77%) had a pet cat or dog. The concentration of mite allergen was very low (mean 0.18 micrograms Der p milligrams sieved house dust), whereas that of cat allergen was high in homes with a cat (80.8 micrograms Fel d milligrams) but also in homes with no cat (3.2 micrograms Fel d milligrams). The results show that in a mite-free environment with low levels of outdoor air pollution, asthma was still a major cause of morbidity among schoolchildren.(ABSTRACT TRUNCATED AT 250 WORDS)