To assess the incidence and effects of continuing gluten ingestion in coeliac disease 51 adult coeliac patients were studied after four to 132 (mean 63) months on a prescribed gluten-free diet. Each patient completed a prospective dietary questionnaire, underwent a repeat jejunal biopsy, and gave serum for gluten antibody estimation. Altogether 65% of patients were still ingesting gluten, often inadvertently. Direct questioning on dietary habits had failed to uncover most of this consumption. The gluten antibody test proved a useful screening test for detecting continuing gluten ingestion and patients with both persistent subtotal villous atrophy and gluten antibodies were almost certain to be taking large amounts ( more than 2 g/day). The presence of persistent partial villous atrophy was found, however, to be an unreliable guide to gluten intake.