Immunological factors play in recurrent oral ulceration (ROU) and Behçet's syndrome (BS) but these cannot be distinguished by means of antibodies and cell-mediated immune responses to oral mucosal antigens. However, HLA markers seem to differentiate ROU from BS. Immune complexes have now been found in about 55% of patients with BS and fewer patients with ROU, so that the transition from the focal ROU to the multifocal BS might be mediated by immune complexes. Immunopathological studies have shown that a vasculitis is the essential lesion in BS and this might be secondary to immune complexes inducing complement activation and damage. It is not clear at present whether the increased levels of C9 and C-reactive protein are a manifestation of acute phase reactants. C9 could be involved in complement dependent lysis and C-reactive protein in modulating the cell-mediated immune responses by its effect on T lymphocytes. It seems that at least two types of damaging immune mechanisms have been identified; cell-mediated and immune complex induced reactions and these might account for the association between ROU and BS.