Direct immunofluorescence (IF) investigations in oral aphthae were performed in 17 patients with recurrent aphthous ulcers (RAU) and in 9 patients with Behçet's disease (BD). In addition, cutaneous hyperreactivity lesions from 2 BD patients were examined. The results were compared to direct IF findings in 28 patients with nonaphthous oral diseases. There was no difference between direct IF on oral aphthae in RAU compared to BD. Deposition of C3 in vessel walls of the subepithelial connective tissue was observed in 13 of 17 patients with RAU and in all patients with BD. IgM deposits in vessel walls were found in 5 RAU and 3 BD patients. Deposition of C1q or C4 was not present. By contrast, IgG and/or IgM, C1q, C3 and C4 were detected in dermal vessel walls of skin hyperreactivity lesions from 2 BD patients. This finding is considered to be a true immune complex vasculitis. Compared to nonaphthous oral lesions, oral aphthae of BD and RAU were characterized by C3 deposition in the subepithelial vessel walls.