T-cell lymphoma: A disease in which lymphoid cells called T-cell (or T-lymphocytes) becomes malignant. T-cell lymphomas account for a minority (about 15%) of non-Hodgkin lymphomas in the US. T-cell lymphomas are highly diverse and include the following 1. Anaplastic large cell lymphoma (ALCL), which can be primary cutaneous ALCL or systems ALCL 2. Lymphoblastic lymphoma (mainly in children and adolescents) 3. Mycosis fungoides (called Sezary's syndrome if the malignant T-cells circulate in blood) 4. Peripheral T-cell lymphoma (a heterogenous group of generally aggressive diseases) T-cell lymphomas account for about half of pediatric lymphomas