Immunoglobulin E (IgE)-dependent mechanisms play a pivotal role in mediating allergic disease. Previously, VH-Cepsilon transcripts from blood or spleen of atopic asthmatics have been analysed for VH gene usage and patterns of somatic mutation. An over-representation of the minor VH5 family has been observed, consistent with a superantigen drive. As local mucosal events in IgE production may be more significant in the disease process, we have analysed VH-Cepsilon transcripts from a bronchial biopsy of a patient with severe asthma. VH5 predominance was confirmed with 10 of 30 unique clones derived from this family. Repeated sequences, some with intraclonal variation, revealed clonal expansion and continuing mutational activity at the site. Unexpectedly, three unmutated VH-Cepsilon sequences were found, indicating that isotype switching to IgE can occur without mutation. Detection of a sister clone with extensive mutations was again consistent with local mutational activity. Evidence for local isotype switching was obtained by identification of clonally related immunoglobulin M (IgM), immunoglobulin G (IgG) and immunoglobulin E (IgE) sequences. However, in contrast to findings in blood, no IgG4 transcripts clonally related to IgE were detected, suggesting that the balance between synthesis of IgG4 and IgE may differ between systemic and local sites. These data confirm a VH5 bias in IgE, and support the concept that IgE-synthesizing B cells arise via local differentiation.