Asthma is the most common chronic disease in industrialized nations, affecting more than10 million people in the U.S. alone. Familial aggregation and concordance rates in monozygotic twins have suggested a genetic component to asthma. Dr. Ober and colleagues have been conducting studies on the genetics of asthma and atopy in the Hutterites, an inbred population of European origins that practices a communal lifestyle. A genome-wide screen with 564 markers (average spacing 6 cM) was completed in an extended pedigree of 717 Hutterites who were well characterized with respect to asthma, atopy, and related phenotypes. These individuals are descendants of only 64 ancestors who lived in the early 1700's to the early 1800's. Evidence for linkage with bronchial hyperresponsiveness (BHR) by the likelihood ratio test extended over 30 centimorgans (cM) on chromosome 5p, with P-values as small as 0.001. Additional evidence for linkage at this same location was evident by the transmission disequilibrium test (P=0.0061). Typing additional markers in this region identified a critical region of 2.4 cM, corresponding to 1.5 Mb of DNA, and a high risk haplotype that is over transmitted to affected individuals.