Intestinal permeability is frequently abnormal in patients with celiac disease. The long-term effect of a gluten-free diet on intestinal permeability and the correlation of intestinal permeability with a gluten-free diet are not known. The objectives of this study were to determine the responses of intestinal permeability and antibody testing to gluten free diet and the degree of correlation of these measurements with gluten ingestion. In this prospective study, patients with celiac disease were divided into three groups based on length of time on a gluten-free diet: Group A, < 1 month; Group B, 1 month-1 year; Group C, > 1 year. Patients in Groups B and C were tested at baseline and at 4-12 weeks later for the following: lactulose/mannitol intestinal permeability, endomysial antibody, and 3-day food record. Permeability tests were also performed in Group A and control subjects. Intestinal permeability was elevated in newly diagnosed celiac disease and in individuals on a gluten-free diet for less than 1 year. Intestinal permeability was normal in 80% at visit 1 and 87% at visit 2 in individuals with celiac disease on a gluten-free diet for more than a year. Trace gluten ingestion was associated with increased intestinal permeability on visit 2 (P = 0.0480). The sensitivity of detecting gluten ingestion as measured by a 3-day food record was higher for permeability testing (29 and 36%) compared with endomysial antibody testing (18 and 18%) for visits 1 and 2, respectively. Intestinal permeability normalizes in the majority of individuals with celiac disease on a gluten-free diet. Gluten ingestion as measured by a 3-day food record correlates with intestinal permeability measurements. The role of permeability testing in the follow-up of patients with celiac disease warrants further investigation.