Treatment of celiac disease (CD) is based on the complete avoidance of gluten-containing products in the diet. However it is not known whether tiny amounts of gluten are harmful for patients on long-term treatment. This is an important issue, as even a strict gluten-free diet (GFD) is usually contaminated by traces of gluten, e.g. in wheat starch and processed food. The aim of this study is to investigate the toxicity of the prolonged ingestion of gluten traces in treated CD patients. This is a prospective, placebo-controlled, double-blind study. Patients are adults with biopsy-proven CD on treatment with the GFD for at least 2 years. During the study period their background daily gluten intake is kept lower than 5 mg. After baseline evaluation, patients continue their GFD and are assigned to ingest 0 mg, 10 mg or 50 mg of daily gluten (incorporated in a capsule) for 90 days. The following evaluations are performed both at baseline and after the micro-challenge: clinical, serological (anti-transglutaminase and antigliadin antibodies), and small intestine histology.