The ultrastructure of oral lesions in 6 patients with erythema multiforme (EM) and of one apparently clinically healthy oral mucosa in one patient with recurrent EM during remission is described. Alterations were observed in epithelium, basal lamina and lamina propria. Both intercellular and intracellular oedema, intracellular vacuolization, decreased numbers of desmosomes, and also loss of cytoplasmic organelles and occasional nuclei were noted in the epithelium. Inflammatory cells--mainly lymphocytes--were found intra-epithelially. Several discontinuities together with some evidence of duplication of the basal lamina were seen in five of the six lesional mucosa specimens. The inflammatory infiltrate in the lamina propria consisted mainly of lymphocytes, although plasma cells, neutrophilic and eosinophilic leukocytes, macrophages and mast cells were also found. Some of the mast cells were partly degranulated. The apparently clinically healthy oral mucosa in the patient with EM in remission showed mild inflammatory changes. The changes observed in the lesional mucosa in EM are thus according to our study mostly non-specific inflammatory alterations and are not pathognomonic for EM.