We evaluated hepatic T lymphocyte phenotypes in a dog with chronic hepatitis. Before treatment, numerous CD3+ lymphocytes were demonstrated in the liver, and the ratio of CD4+/CD8+ was remarkably high (2.96; reference range, 0.33+/-0.12). After treatment, CD3+ lymphocyte infiltration in the liver was reduced, and the ratio of CD4+/CD8+ decreased to 0.31. Therefore, hepatic T lymphocytes, especially CD4+ lymphocytes, might play a central role in the pathogenesis of this dog with chronic hepatitis.