Cytogenetic, pathologic, and immunologic studies were done on 10 patients with lymphoblastic lymphoma (LL). The median age was 21 years, and males predominated in a ratio of 9:1. At the time of diagnosis, 6 of the 10 patients had an anterior mediastinal mass, 3 had bone marrow involvement, and 2 had central nervous system involvement. All patients were treated with intensive combination chemotherapy. The median survival of the 8 patients who died (7 of whom were adults) was 10.5 months; all 8 achieved only a partial remission. Two other patients were children; they continue in complete remission. Immunologic marker studies were done on 9 of the patients; 5 had T-cell LL, and the other 4 had non-T, non-B LL. In a study of T-cell subpopulations with the use of monoclonal antibodies, the malignant T cells of the 2 patients tested appeared to be mature thymocytes. Clonal chromosome abnormalities were seen in involved tissues from 8 patients; the other 2 patients had a normal karyotype. A 9q+ chromosome, a 6q- chromosome, and a 1q-chromosome associated with 1q trisomy were each seen in 2 patients. No patients had a 14q+ chromosome, which is the most common abnormality in various lymphoproliferative disorders, including other T-cell malignant diseases. The variable chromosome pattern in LL could be related to the heterogeneity of the immunologic phenotype of the malignant T cells. More data are needed before it can be established whether there are any correlations between a particular karyotype and the immunologic phenotype of LL cells.