Celiac disease (CD) may be found in association with other autoimmune diseases. We investigated the relation between autoimmune hepatitis (AIH) and CD by assessing the prevalence of IgA and IgG anti-tissue transglutaminase (tTG) antibodies in AIH, and by verifying whether the findings were associated with clinical and histological features of CD. Forty-seven consecutive patients with AIH (type I: n = 39; type II: n = 8) were studied. One hundred patients with chronic hepatitis C, and 120 healthy blood donors were also studied as controls. We analyzed sera for the presence of IgA and IgG anti-tTG antibodies using a specific human recombinant tTG immunoenzymatic assay. Anti-tTG positive patients and controls were further tested for anti-endomysium antibodies (EMA) and HLA typing, and those found positive by either of these tests underwent duodenal biopsy to confirm a possible diagnosis of CD. Three of the 47 AIH patients (6.4%) were positive for IgA anti-tTG and EMA antibodies, and were subsequently confirmed to be affected with CD by small-bowel biopsy findings. No IgG anti-tTG positivity was found in the AIH patients. None of the controls were positive for IgA anti-tTG, and only one with chronic hepatitis C had a low positive reaction for IgG anti-tTG, which resulted as a false positive. The crude prevalence rate of CD in AIH was 63.8 per 1,000 (95% CI, 13.2–186.1), and it was significantly higher than that found in the general population in Italy (4.9 per 1,000; 95% CI, 2.8–7.8). The results of this study showed a high prevalence of CD in patients with AIH. For this reason, early serological screening testing for CD is strongly recommended for all AIH patients. J. Clin. Lab. Anal. 19:6–10, 2005. © 2005 Wiley-Liss, Inc.