Oral lesions and mucosal inflammatory changes may appear in dermatitis herpetiformis (DH). We examined whether potassium iodine, known to initiate blisters in the DH skin, or wheat gliadin, responsible for T-cell-dependent intestinal damage, can induce visible or microscopic changes in oral mucosa. Six patients with active DH were challenged with crude gliadin and 50% potassium iodine applied in patch test chambers on buccal mucosa for 12 h. After reading, biopsies were taken from the challenged and non-challenged mucosa. No macroscopic or microscopic vesicles were seen. However, gliadin- but not iodine-challenged epithelium showed increased numbers of CD4+ lymphocytes in all 5 patients with representative specimens (p = 0.06). No marked changes were found in the numbers of CD8+ or TcR alpha/beta+ lymphocytes, and the numbers of TcR gamma/delta+ cells remained at a low level. The results show that oral mucosa is resistant to production of macroscopic or microscopic DH lesions. It is, however, capable of reacting to locally applied gliadin by a T-cell response consisting of CD4+ lymphocytes.