We evaluated B lymphocyte function in 23 patients with Behçet's disease at various stages. The patients with active disease, but not those with inactive disease, were found to have elevated numbers of cells spontaneously secreting immunoglobulin and a decreased B cell response to the T cell-independent B cell mitogen, Staphylococcus aureus Cowan 1. Moreover, B cells from almost all patients with Behçet's disease were unresponsive to the T cell-dependent polyclonal activator, pokeweed mitogen. These results indicate that B cell abnormalities, including some which are associated with disease activity, could be involved in the pathogenesis of Behçet's disease.