Aim:This study was performed to evaluate the diagnostic value of hepatocyte-derived microRNA (miRNA)-122 in acute and chronic hepatitis of dogs.Materials and Methods:A total of 26 dogs presented at Veterinary Teaching Hospital, Faculty of Veterinary Medicine, Cairo University, 16 dogs out of 26 showing clinical signs of hepatic insufficiency were subjected to clinical, ultrasonographic, hematobiochemical and ultrasound-guided fine-needle biopsy for cytological and histopathological investigations. On the basis of these results, 7 dogs out of 16 dogs were found to be suffering from acute hepatitis and 9 dogs suffering from chronic hepatitis. 10 clinically healthy dogs were kept as control. Serum hepatocyte-derived miRNA-122 was analyzed by real-time quantitative polymerase chain reaction in all dogs.Results:The dogs suffering from acute hepatitis manifested jaundice, vomiting, and depression while dogs with chronic hepatitis manifested anorexia, abdominal distension, weight loss, and melena. Hematological parameters showed normocytic normochromic anemia and thrombocytopenia in both acute and chronic hepatitis groups. Alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), and total bilirubin were significantly higher than control values in acute hepatitis. In chronic hepatitis, total protein and albumin were significantly lower than control values with normal ALT, AST, ALP, and gamma-glutamyltransferase values. Ultrasonography revealed a diffuse decrease in hepatic echogenicity in acute hepatitis while the increase in hepatic echogenicity and anechoic ascetic fluid in chronic hepatitis. Cytology revealed hepatic vacuolar degeneration and histopathology revealed necrosis and apoptosis of hepatocyte in acute hepatitis while revealed massive fibrous tissue proliferation in hepatic parenchyma in chronic hepatitis. Serum miRNA-122 analysis, normalized for glyceraldehyde-3-phosphate dehydrogenase expression revealed a significant increase in acute hepatitis accompanied with elevation in ALT and AST, while in chronic hepatitis, elevation of serum miRNA-122 was accompanied with ALT and AST of the normal range.Conclusion:Serum hepatocyte-derived miRNA-122 is of diagnostic value and highly stable blood indicator for the detection of hepatocellular injury in dogs than aminotransferases, especially in cases where aminotransferases do not exceed normal serum level.