The β–subunit of the high–affinity IgE receptor (FcεRI–β) on chromosome 11 is maternally linked to atopy, the state of enhanced IgE responsiveness underlying allergic asthma and rhinitis. We have identified a common variant of FcεRI–β, lle181Leu within the 4th transmembrane domain. Leu181 shows significant association with positive IgE responses in a random patient sample. Amongst 60 unrelated nuclear families with allergic asthmatic probands, Leu181 is identified in 10 (17%), is maternally inherited in each, and shows a strong association with atopy. Our data indicate that FcεRI–β, subject to maternal modification, may be the atopy–causing locus on chromosome 11q.