We report a new methodology for the long-term growth of malignant T-lymphoblasts from patients with T-cell acute lymphoblastic leukemia (T-ALL) and T-cell lymphoblastic lymphoma (T-LL). When malignant cells were cultured in the presence of insulin-like growth factor I under hypoxic conditions, cellular proliferation occurred that resulted in the establishment of immortal cell lines from ten of 12 patient tumors. Authenticity of each cell line was verified by a direct comparison of the immunophenotype, karyotype, and immunogenotype with the patient's tumor cells. This improved method of cell culture permits frequent establishment of cell lines from patients with T-ALL/T-LL, thereby aiding in analysis of thymocyte transformation and neoplasia.