Do subjects living in high altitude where house-dust mites are known to be uncommon exhibit a lower prevalence of asthma and allergy to house-dust mites? To answer this question, we compared the prevalence rates of asthma and skin reactions to house-dust mites in two towns with contrasted environments: Marseille, located on the seashore, and Briancon, 1350 m in altitude. The study population consisted of a random sample of 4,008 people in Marseille and 1,055 people in Briancon. All subjects received a home questionnaire, and a sample of patients and asymptomatic subjects had a skin-prick test evaluation. The cumulative prevalence of asthma was equal to 4.1 percent in Marseille and 2.4 percent in Briancon, a difference which was significant (p = 0.01). The prevalence of positive skin tests to housedust mites in asymptomatic subjects was equal to 27.5 percent in Marseille and 10.2 percent in Briancon (p less than 0.001). This study supports the hypothesis that exposure to environmental factors may have a major influence on developing allergic diseases.