A cohort of 1333 subjects, aged 3 years and older, was followed for a mean of 8.1 years to assess changes in allergen skin test reactivity. The overall prevalence of reactivity to the five antigen mixtures was 39.1% during the initial survey and 50.7% after the follow-up period. The greatest increase in prevalence occurred among children and teenagers (22.2% and 19.5%) with only minimal increases after the age of 65 years (6.0%). No difference in prevalence between male and female subjects was apparent, either initially or at the end of the follow-up period. In-migration to the Tucson area was a major factor in determining changes in reaction prevalence. Among subjects more than 35 years of age, recent in-migrants accounted for most of the increased prevalence. Comparisons of atopy among consistent smoking groups confirmed the previous observation that smokers are less atopic than either nonsmokers or exsmokers, probably because of a self-selection process. In contrast, exsmokers were generally the most atopic, both initially and at the end of the longitudinal observation period. The high overall prevalence of allergen reactivity in this population is believed to be due in large measure to high year-round concentrations of multiple aeroallergens in the Tucson environment.