BACKGROUND & AIMS: Immunoglobulin A (IgA) autoantibodies to endomysium (EMA) are highly specific and sensitive markers for celiac disease. Recently, we identified tissue transglutaminase (tTG) as the major if not sole endomysial autoantigen. METHODS: An enzyme-linked immunosorbent assay (ELISA) was established to measure IgA anti-tTG titers in serum samples from 106 celiac patients with partial or subtotal villous atrophy, 43 celiac patients on a gluten-free diet, and 114 diseased and healthy controls. Results were correlated with clinical and histological data and with EMA titers. RESULTS: In patients with biopsy-proven celiac disease consuming a normal, gluten-containing diet, 98.1% of the serum samples had elevated IgA titers against tTG, whereas 94.7% of the control sera were negative. IgA anti-tTG correlated positively with semiquantitative IgA EMA titers (r = 0.862; P < 0.0001). CONCLUSIONS: An ELISA based on tTG allows diagnosis of celiac disease with a high sensitivity and specificity. IgA anti-tTG and IgA EMA show an excellent correlation, further confirming the enzyme as the celiac disease autoantigen. Because the assay is quantitative, not subjected to interobserver variation, and easy to perform, it will be a useful tool for population screening of a hitherto underdiagnosed disease.