T-cell lymphoblastic lymphoma is an aggressive malignancy that represents 85% of all lymphoblastic lymphomas. It usually occurs in late childhood, adolescence and young adulthood with a 2:1 male preponderance and it presents with pleural effusion and respiratory symptoms and in rare cases vena cava syndrome can be encountered. We present the case of a 13-year-old patient who was referred to our clinic from a local hospital where he was diagnosed with a mediastinal tumor. The patient presented with thoracic pain, fever, coughing and fatigability for a month prior to admission, after having underwent surgery for abdominal pain (appendectomy). On admission to our hospital, a thoracic computed tomography (CT) scan was performed and showed the presence of an anterior mediastinal mass measuring 109/76/140 mm, well defined, which came in close contact with the superior vena cava, the ascending aorta and the pulmonary artery, right pleural effusion and a collapsed lung on the right side. The decision was taken to perform a tumor biopsy and a right pleural drain was placed. The patient's post-operative evolution was favorable with the remission of the respiratory symptoms. The histopathological result showed the presence of T-cell lymphoblastic lymphoma and the patient was then transferred to the oncology ward where he underwent chemotherapeutic treatment, with a favorable outcome. T-cell lymphoblastic lymphoma is an aggressive type of lymphoma and it is usually hard to diagnose considering the fact that the symptoms are often vague. It is essential to establish the diagnosis without delay and start appropriate chemotherapeutic treatment.