A correlation between the titre of circulating IgG autoantibodies and disease activity has been difficult to demonstrate in bullous pemphigoid. We postulate that isotype switching from "inflammatory" IgG1 to "blocking" IgG4 subclass antibodies might contribute to disease remission. We studied 16 patients with bullous pemphigoid, 3 patients with cicatricial pemphigoid and 2 patients with epidermolysis bullosa acquisita at different stages of the disease. The titres of IgG subclass and total IgG basement membrane zone autoantibodies were correlated with clinical activity. Ten of the 16 bullous pemphigoid patients went into remission. The total IgG autoantibody levels showed variable changes. IgG1 autoantibody decreased in 7 patients (3 were unchanged) and IgG4 autoantibody increased in 9 patients. The 6 patients with clinical activity did not show such changes in IgG1 and IgG4 autoantibodies. Similar results were observed with the other bullous diseases. Our data suggest that isotype switching from "inflammatory" IgG1 to "blocking" IgG4 antibody correlates with improvement in bullous pemphigoid.