Diffuse large B-cell lymphoma (DLBCL) is the most common non-Hodgkin lymphoma comprising a heterogeneous group of disorders with variable histological and clinical behavior. Although other lymphomas may present in the leukemic phase more frequently, this appearance is unusually observed among DLBCL cases. Diagnosing lymphoma is not always easy, and the patient’s clinical status quite often may hamper invasive procedures for diagnosis pushing the clinician to look for alternatives to reach the nearest possible accurate diagnosis. The authors report the case of a middle-aged man who presented the history of malaise, weight loss, and low-grade fever. The peripheral blood count showed leukocytosis with the presence of blasts and thrombocytopenia. The cytological morphology and immunophenotyping of the peripheral blood and bone marrow aspirate, as well as the bone marrow biopsy accompanied by a thorough immunohistochemical analysis, rendered the diagnosis of DLBCL in the leukemic phase. The patient was prescribed R-CHOP with a favorable outcome. Intra-abdominal lymph node biopsy was avoided because of the patient’s critical medical condition. The authors highlight this rare form of presentation of DLBCL as well as the combination of peripheral blood, bone marrow aspirate, and bone marrow biopsy for reaching the diagnosis in cases were a lymph node sample is unavailable for the diagnostic work-up.