This study describes the clinical manifestations, hematological and biochemical changes as well as postmortem findings of thymic lymphoma in three young cattle with cervical swelling. Anorexia, weight loss, distended jugular veins, difficult breathing, severe tympany and ataxia were the main observed clinical signs. The results of the indirect enzyme-linked immunosorbent assay indicated that all animals were seropositive for bovine leukemia virus infection. Fine needle aspiration biopsy of cervical mass revealed that numerous lymphocytes (typical and atypical) are apparent. Hematologoical examinations indicated leukocytosis with severe lymphocytosis, while biochemical analysis showed hyperglycemia, increased concentrations of non-esterified fatty acids and hydroxy butyric acid, and increased serum activity of aspartate aminotransferase and lactate dehydrogenase. Due to poor prognosis, the three cattle were euthanized and postmortem examination was thoroughly carried out and revealed presence of tumor in both cervical and thoracic thymic gland, cardiomyopathy, distended hepatic veins congested lung and hemothorax. The current study concluded that thymic lymphoma must be considered in the differential diagnosis in heifers with cervical swelling, severe tympany, and jugular venous distention.