Leptin, the adipocyte hormone, and its receptor have been implicated in the differentiation/proliferation of hematopoietic cells. Given that the deregulated expression of a variety of growth factors and/or their receptors has been implicated in the pathogenesis of certain leukemias, we aimed to characterize the potential differences in the expression pattern of the two major leptin receptor transcript variants in peripheral blood mononuclear cells (PBMC) between different hematologic malignancies. Using RT-PCR and Southern blotting, we compared the expression levels of the two major leptin receptors, the longest (OB-RL) and the shortest (OB-RS) splice variants, in PBMC from patients with myelodysplastic syndrome (MDS), acute myeloid leukemia (AML), healthy individuals and two human hematopoietic cell lines (HL-60 and K562).Expression of the OB-RS transcript clearly exceeded that of OB-RL in all patients and controls and in the HL-60 cells, but this was reversed in the K562 cell line. However, the expression of the OB-RL was significantly lower in MDS compared to controls and tended to be so in AML, while OB-RS tended to be higher in MDS and AML patients compared to controls, but this difference was not significant.Serum leptin levels and circulating soluble leptin receptor levels were slightly but not significantlyhigher in AML and MDS. These alterations in the expression of the leptin receptor isoforms in MDS and AML patients could suggest a potential role of leptin and its signaling in hematopoietic malignancies, which requires further examination.