Serum IgG, IgM, IgA, IgD and IgE concentrations were examined in 26 patients with Behcet's syndrome, 70 patients with recurrent aphthous stomatitis and 56 healthy controls. IgA concentrations, but not IgG, IgM, IgD or IgE, were significantly raised in Behcet's syndrome compared with controls. Serum IgD and IgE concentrations but not IgA, IgG, or IgM were significantly greater in recurrent aphthous stomatitis than in controls.