To assess the prevalence and distribution of allergic skin-test reactions in a general population sample, allergy prick tests were applied to 3101 subjects older than 2 years of age. Test materials included allergens common to the Tucson environment, and subjects were randomly stratified by age, sex, and socioeconomic status. No difference in the prevalence of measurable reactions was found among male subjects versus female subjects. A definite age relation was apparent, however, with the peak prevalence of reactivity (more than 40%) occurring during the third decade, and falling rapidly past age 50. When present, reactions tended to be multiple, highly reproducible, and more frequent among those in the higher socioeconomic strata. The prick test was judged to be a useful tool for the assessment of atopy.