The diagnosis of acute leukemia is based on a combination of clinical, hematological, morphological, cytogenetic, and immunophenotypic data. The authors report a case of reactive lymphocytosis with extremely elevated lymphocytic and lymphoblastic leukocytosis that mimicked acute lymphoblastic leukemia, not only morphologically, but also in immunophenotypic analysis. They could not determine any underlying disease marker other than infectious symptoms that were present for 20 days prior to presentation to their clinic. Although this case presented with extremely high lymphocytic leukocytosis, the patient had normal blood cell lineage, a moderate level of blastic cells in bone marrow, and normal physical findings. These findings convinced the authors to follow up the patient before beginning treatment. Copyright © Informa Healthcare.