Reactive leukocytosis has been reported in patients with non-Hodgkin's lymphoma of different histologic types. On the other hand, the blastic crisis of chronic myelocytic leukemia (CML) can sometimes be localized outside the bone marrow and simulate lymphoma, particularly when the blasts are of lymphoid lineage and the blastic crisis is the presenting feature of the disease. We report two patients in whom the differential diagnosis between lymphoblastic lymphoma with reactive leukocytosis and blastic crisis of CML outside the bone marrow was raised. They were two males aged 32 and 22 years, respectively, with lymphadenopathy (and one with splenomegaly), who were initially diagnosed of T lymphoblastic lymphoma. In both cases, leukocytosis was detected with myelemia and dysgranulopoiesis in the onset in one of them and when lymphadenopathy reappeared after remission in the other one. In addition, one patient had marked eosinophilia. In the bone marrow there was marked granulopoietic hyperplasia, with a reduction of fatty cells, and the granulocyte alkaline phosphatase index was reduced. However, the cytogenetic study did not disclose the existence of Philadelphia (Ph) chromosome, and bcr/abl molecular rearrangement was also not observed in the molecular study of both cases. We discuss the basic aspects of differential diagnosis between T lymphoblastic lymphoma with leukemoid reaction and T lymphoid lymphadenopathic blastic crisis of Ph-negative, bcr/abl-negative CML.