The effect of topical recombinant interferon alfa 2c hydrogel (IFN alpha 2C) in the aphthous lesions of the mouth in Behcet's syndrome was assessed in twenty patients in a twelve-week open trial. IFN alpha 2C applied to the mouth for four weeks significantly reduced the number of aphthae in the post-treatment phase compared to the pretreatment and treatment phases. No side effects were recorded. Topical IFN alpha 2C seems to be effective in the treatment of the aphthae in Behcet's syndrome.