Clinical and cytogenetic studied were done on 12 patients with lymphoblastic lymphoma. The ages of the patients ranged from 17 to 58 years (median, 24.5 years). Males predominated by a ration of 5:1. Ten of the 12 patients had a mediastinal mass at diagnosis; two thirds of the patients had involvement of the bone marrow (3 of 12 patients initially were in a leukemic phase), and 5 of 12 patients had involvement of the central nervous system (CNS) with lymphoma. In all patients, the malignant cells were characterized by nuclear convolutions. Cytogenetic studies done on four patients showed clonal chromosome abnormalities in two cases. Six of the 12 patients were treated with the combination chemotherapy regimen COPA and received CNS prophylaxis; their survival was slightly longer than that of the other patients, who were treated with a variety of treatment regimens. Two of the 12 patients achieved a complete remission. The median survival of all patients was 11.5 months but was shorter in patients with initial CNS involvement or leukemia. The poor results achieved thus far in the treatment of this disease demand a new therapeutic approach.