Large granular lymphocytic leukemia/lymphoproliferative disorder (LGL-L/LPD) is a heterogeneous neoplastic disease of large granular lymphocytes and is a well-known cause of cytopenias. We aimed to reveal the incidence of LGL-L/LPD in patients with cytopenia(s) of unknown etiology (CUE). Twenty-eight patients with CUE were investigated for LGL-L/LPD. T-cell LGL leukemia (LGL-L) was diagnosed in 12 (42.9 %) patients. The frequencies of LGL-L in patients who had anemia, neutropenia, and thrombocytopenia were 9/14 (64.2 %), 11/23 (47.8 %), and 3/10 (30 %), respectively. Seventeen of the 28 patients met the criteria of idiopathic cytopenia of undetermined significance (ICUS), and LGL-L was found in six (35.3 %) of them. We conclude that LGL-L is a rather common disease in patients with CUE and ICUS. It should be considered in this patient group and investigated thoroughly.