Adult lymphoblastic lymphoma (LBL) is an aggressive form of non-Hodgkin lymphoma occurring in predominantly adolescent and young adult men, accounting for 1% to 2% of all non-Hodgkin's lymphomas. In contrast to B-LBL, T-cell LBL is much more common, accounting for up to 90% of disease in adults. Mediastinal mass, pleural and/or pericardial effusions are the major characteristics of T-LBL. We report an 18-year-old male with a pleural effusion, mediastinal mass, a light pericardial effusion, and a normal hemogram. The cytology of the pleural effusion initially suggested malignancy, but definitive diagnosis was unclear. After a medical thoracoscopy, the partial pleura was picked and immunophenotypic study revealed the following: CD3(+), TdT(+), CD99(+), CD20(-). The patient was finally diagnosed with T-LBL and died only 6 months after that. The case highlight the point that medical thoracoscopy is a safe and accurate diagnostic procedure for pleural diseases, and partial pleura biopsy with immunophenotyping was essential for achieving the correct diagnosis of LBL.