Previous investigations in alpine altitudes suggest a very low etiopathogenic relation between house-dust mites and bronchial asthma in these geographical zones. Our purpose was to analyze the relationship between asthma and sensitization to the house-dust mites D. pteronyssinus and D. farinae in Andean altitudes. We studied 587 asthmatic patients, residents in Quito, Ecuador, (2800 m above sea level), ranging in age from 8 months to 84 years. The patients were divided into five age groups. We investigated the prevalence of house-dust mite sensitization by skin prick tests in each group. Fifty asymptomatic students with ages between 6 to 20 years old were studied as a control group. Three hundred and thirty asthmatic patients (56.2%) were sensitized to house-dust mites while only 15 (30%) of the 50 asymptomatic students were sensitized (p < 0.01). The percentage of asthmatics sensitized followed an age-dependent curve that began with the children under 6 years old (45.1%), reached its maximum between 12 and 19 years old (79.0%) and decreased to 25.8% in asthmatics older than 40 years. The differences among these groups were statistically significant (p < 0.01). Also, we observed a significant prevalence in sensitization (p < 0.01) in males versus females. The mean size of the skin reaction obtained with D. pteronyssinus and D. farinae was larger (p < 0.001) in the 12-19 year olds than in the group of asthmatics over 40 years old. It was also greater in the asthmatics with chronic rhinitis than in the group of asthmatics without nasal symptoms (p < 0.01). We suggest that the house-dust mites D. pteronyssinus and D. farinae are an important source of allergens in the pathogenesis of bronchial asthma in patients who live in the high Andes.