This work is devoted to the analysis of the nature of lymphoblastic lymphoma/leukemia with convoluted nuclei which were initially described by Barcos and Lukes. Ultrastructural, cytochemical, and immunologic features of tumor cells were investigated in patients chosen according to known morphologic criteria. Through results of the E rosette test, the patients were divided into two groups (E+ and E-). In the E+ group, the predominant features were sex (only men), the mediastinal localization, and the focal positivity of the acid phosphatase reaction. Cytotoxicity tests with rabbit antihuman T-lymphocyte anti-serum confirmed the results of the E rosette test in the 3 patients of the E+ group who were tested and were also positive in 2 patients from the E- group (1 of these 2 patients had the characteristics found in the E+ group and can thus be related to this group; the other patient had none of these characteristics). This raises the question of a leukemia arising from a less differentiated T-cell but this interpretation is limited by the specificity of the anti-T-serum. Ultrastructural study defines more precisely the convoluted aspect but does not at present allow a distinction between the two groups.