One hundred and thirty-two different intestinal alpha 1-antitrypsin clearance tests were performed in 48 untreated adult celiac patients, 64 patients taking a gluten-free diet, and 20 adult healthy controls. In the untreated group, 95% of patients had enteric protein loss with values higher than the upper limit of normality (mean +/- 2 SD). In the treated group of patients, only 22% had abnormal levels of alpha 1-antitrypsin clearance. Sixteen patients who had elevated clearance before treatment had decreased clearance after an average of 7.4 months on a gluten-free diet. There was a significant relation (p less than 0.05) between the alpha 1-antitrypsin clearance and the degree of alteration of the jejunal histological structure. We conclude that enteric protein loss is a very frequent finding in celiac patients and the measurement of alpha 1-antitrypsin clearance may be a reliable method to evaluate the activity of the disease and useful in following the efficacy of treatment.