Summary. Asthma is a manifestation of bronchial hyperreactivity (BHR) and forms part of the spectrum of atopic diease. Some pedigree studies of atopy have suggested linkage with the high-affinity IgE receptor (Fc∈ Rlβ) gene on chromosome 11q13, but others find no linkage. The molecular genetics of asthma and BHR have not been studied in the general population. We examined the genetic linkage of the Fc∈ Rlβ gene with clinical asthma and the underlying phenotypes of BHR (to methacholine) and atopy (defined by skinprick testing) in 123 affected sibling-pairs recruited from the general population. We found evidence of significant linkage of a highly polymorphic microsatellite marker in the fifth intron of the Fc∈ Rlβ gene to a diagnosis of asthma (18·0% excess of shared alleles, p=0·002) and to BHR (21·7% excess of shared alleles, p=0·001). Significant linkage was also observed in siblings sharing BHR when those with atopy were excluded (32·8% excess of shared alleles, p=0·004). Atopy in the absence of BHR did not show significant linkage to the Fc∈ Rlβ gene (7·2% excess of shared alleles, p=0·124). These findings suggest that mutations in the Fc∈ Rlβ gene or a closely linked gene influence the BHR underlying asthma, even in the absence of atopy.