In this study, we tested the serum levels of soluble CD23 (sCD23) in 27 bullous pemphigoid (BP) patients and compared them with the disease activity. Soluble CD23 is the cleaved portion of the low affinity Fc receptor for IgE (Fc epsilon RII/CD23) which has an affinity for IgE and regulates IgE synthesis. Although bullous pemphigoid (BP) is a subepidermal blistering disease characterized by IgG class autoantibodies against the basement membrane of stratified squamous epithelia, several IgE-related phenomena have been reported. Recently, we have shown that Fc epsilon RII-expressing and IgE-bearing cells are detectable in the lesional skin and concluded that an IgE-Fc epsilon RII/CD23 system may be involved in the pathogenesis of this disease. The serum level of sCD23 in BP patients was significantly higher than healthy controls (p < 0.01). In 11 out of 12 patients, the alteration of serum sCD23 levels correlated well with the disease activity. Thus the serum level of sCD23 is useful as a new parameter for assessing the level of disease activity in BP. High levels of sCD23 may represent part of an IgE-mediated immune reaction which may play a role in the pathogenesis of BP.