AIM To evaluate the relationship between serum adenosine deaminase(ADA) levels and histological features in patients with autoimmune hepatitis(AIH).METHODS A total of 80 subjects(52 AIH cases and 28 healthy controls) were included in the study. Patients were diagnosed according to the simplified criteria suggested by the International Autoimmune Hepatitis Group. All of the cases had been diagnosed with AIH between 2010-2015 at Hacettepe University, Department of Gastroenterology. Serum blood samples were collected and stored at-80 ℃ until the biochemical estimation of ADA activity. The diagnosis of patients was confirmed by liver biopsy. Serum ADA > 20 U/L was considered to be high level.RESULTS Mean serum ADA levels were significantly higher in AIH patients than those in healthy controls(25.4 ± 9.6 U/L vs 12.8 ± 2.2 U/L, P < 0.001). Serum ADA levels > 20 U/L were found in 63.5% AIH patients and in 0% healthy controls(P < 0.001). Mean serum ADA levels were significantly increased in each stage of histological activity: 15.2 ± 3.5 U/L for patients with mild interface hepatitis, 23.1 ± 10.0 U/L for patients with moderate interface hepatitis and 30.9 ± 7.0 U/L for patients with severe interface hepatitis(P < 0.001). Correlation analysis showed that there was a positive association between serum ADA levels and histological activity(r = 0.71, P < 0.001). Receiver operating characteristic analysis suggested that 24.5 U/L was the optimum cut-off point of ADA level for severe interface hepatitis(sensitivity 88%, specificity 85.2%, area under thecurve: 0.88).CONCLUSION Because of the positive correlation with inflammatory activity, serum ADA level may be a potential biomarker for predicting or monitoring histological activity in patients with AIH. AIM To evaluate the relationship between serum adenosine deaminase (ADA) levels and histological features in patients with autoimmune hepatitis (AIH). METHODS A total of 80 subjects (52 AIH cases and 28 healthy controls) were included in the study. Patients were diagnosed according to the novel criteria suggested by the International Autoimmune Hepatitis Group. All of the cases had been diagnosed with AIH between 2010-2015 at Hacettepe University, Department of Gastroenterology. Serum blood samples were collected and stored at -80 ° C until the biochemical estimation of ADA The diagnosis of patients was confirmed by liver biopsy. Serum ADA> 20 U / L was considered to be high level. RESULTS Mean serum ADA levels were significantly higher in AIH patients than those in healthy controls (25.4 ± 9.6 U / L vs 12.8 ± 2.2 U / L, P <0.001). Serum ADA levels> 20 U / L were found in 63.5% AIH patients and in 0% healthy controls (P <0.001) For each patient with mild interface hepatitis, 23.1 ± 10.0 U / L for patients with moderate interface hepatitis and 30.9 ± 7.0 U / L for patients with severe interface hepatitis (P <0.001) . Correlation analysis showed that there was a positive association between serum ADA levels and histological activity (r = 0.71, P <0.001). Receiver operating characteristic analysis suggested that 24.5 U / L was the optimum cut-off point of ADA level for severe interface Conclusion: The positive correlation with inflammatory activity, serum ADA level may be a potential biomarker for predicting or monitoring histological activity in patients with AIH.