T-lymphoblastic lymphoma/leukemia (T-LBL) is a neoplasm of immature lymphoid cells (lymphoblasts) committed to the T-cell lineage. The disease usually presents as an extramedullary tumor mass in the anterior mediastinum or lymph nodes (T-LBL). A smaller subset of patients present with predominant blood and bone marrow involvement (T-lymphoblastic leukemia [T-ALL]). Patients who present as T-LBL often convert to a T-ALL clinical picture over time. This clinical overlap, as well as overlap in immunophenotypic and molecular features, led the authors of the World Health Organization (WHO) classification to combine T-ALL and T-LBL cases into one category. There are some immunophenotypic and molecular differences between T-ALL and T-LBL, however, that may explain their different clinical presentations. Here we focus on patients presenting as T-LBL.