29 cases of T-cell derived lymphoblastic lymphoma and T-ALL have been analyzed. There is a striking prevalence of the male sex. In the peripheral blood we often find initially an excessive number of white blood cells combined with normal values for the other constituents in about half of the patients; This may be an expression for the rapid occurrence of leukaemia in T-cell lymphosarcoma. In addition to systemic ALL-therapy we performed X-ray irradiation of the mediastinum in 8 of our patients. This yielded to significantly longer first complete remissions. All patients with T-cell LSA/ALL with or without mediastinal mass should be treated in this manner. Cytochemically a strong focal acid phosphatase reaction was found to be acharacteristic of these cells. It has proved to be a screening method for this disease. The cells are T-cell derived and their pattern of surface markers is similar to that found in fetal thymocytes.