Patients with celiac disease produce not only immunoglobulin A (IgA) but also immunoglobulin G (IgG) and M (IgM) antibodies to gluten. Intake of dietary gluten may hence induce local complement activation and mucosal damage. Jejunal tissue sections from adult patients with celiac disease were examined by immunofluorescence with monoclonal antibodies to activation neoepitopes in C3b and the terminal complement complex (TCC). Subepithelial deposition of TCC was observed in 93% of 28 untreated and in 57% of 23 partly treated study subjects. The immunofluorescence staining intensity was well correlated with the serum level of gluten-specific IgG and IgM (but not IgA), the number of mucosal IgG-producing cells, and the degree of villous atrophy. Similar immune deposits were not observed in 5 successfully treated patients with celiac disease, 5 patients with dermatitis herpetiformis without jejunal villous atrophy, and 90% of 21 control patients with histologically normal jejunal mucosa. Gluten challenge increased the amount of subepithelial TCC and produced additional C3b deposition, suggesting recent complement activation. Ingested gluten might thus, via Ig-mediated subepithelial complement activation, damage the surface epithelium in celiac disease and induce compensatory crypt hyperplasia.