Acute lymphoblastic leukemia with maturation (ALLm) has different disease characteristics that does typical ALL. ALLm is characterized by an increased number of mature appearing leukemic cells (>20% of ANCs in the BM) having differentiation beyond the prolymphocyte stage according to light microscopic (LM) examination. It also has a worse prognosis than typical ALL. We have recently experienced a case of ALLm and we report on this case along with a literature review. A 36 year old patient showed lymphoblasts and mature appearing leukemic cells that were counted up to 15.8% and 23.0%, respectively, of the WBCs on bone marrow examination. Despite their mature appearance, these cells showed positivity for Tdt, CD10, CD19 and HLA-DR on the immunophenotypic study. Differentiating the mature-appearing leukemic cells from the hematogones or mature lymphocytes is difficult, and only through immunophenotypic examination is it possible to discriminate ALLm from typical ALL. We suggest performing a leukemic marker study that includes CD38 to effectively differentiate mature appearing leukemic cells from hematogones, especially for the follow up of leukemia with mature appearing cells.