T-cell large granular lymphocytic leukemia (T-LGLL) is an uncommon but probably underdiagnosed disease caused by clonal proliferation of large granular lymphocytes. Diagnosis is typically based on the high number of morphologically characteristic lymphoid cells and finding of an abnormal immunophenotype by flow cytometry. Because of its relatively indolent clinical behavior, observation is often an appropriate therapy. Here we present a case of a 53-year-old male admitted to the hospital because of abdominal pain. Blood examination revealed mild mycrocitic anemia and multiplied lactate dehydrogenase level. Abdominal ultrasound showed splenomegaly of 16 cm, with no lymphadenopathy. Fine needle aspiration of bone marrow revealed hypocellular marrow with 50% of atypical lymphoid cells. There were 81% of atypical medium sized granular lymphocytes with ir-regularly shaped nuclei in peripheral blood, so the cytologic diagnosis was lymphoproliferative proc-ess. Bone marrow biopsy showed nodular and interstitial proliferation of small, partially atypical T lymphocytic cells positive for CD2, CD3, CD5, CD8, granzyme and TIA, and negative for hairy cell markers, CD10, MUM 1, bcl 1, CD4 and CD56. The finding was consistent with T-LGLL. Due to splenomegaly, the patient was treated with cyclosporine and gradually reduced dose of corticoster-oids, leading to regression of splenomegaly and normalization of lactate dehydrogenase level.