Sera from 41 children with untreated celiac disease (CD), and 16 children with dermatitis herpetiformis (DH), and 57 matched controls were studied for serum antibodies to synthetic peptides derived from an early E1b protein of adenovirus type 12 and type 40 (Ad12, Ad40) and A-gliadin. In addition to peptides which share homology with A-gliadin, "nonhomologous" peptides derived from Ad12 and Ad40 E1b proteins were used as antigens in ELISA. Patients with CD and DH had significantly higher IgG antibody levels than those of controls to the nonhomologous Ad40 E1b peptide. Concomitant presence of antipeptide IgA antibodies to A-gliadin and Ad12 E1b or Ad40 E1b peptides (which share no homology with A-gliadin) increased the risk of CD/DH suggesting that both adenovirus infections and gluten exposure contribute to the development of CD/DH.