Labeling index (LI), apoptosis, levels of 2 pro-apoptotic cytokines tumor necrosis factor-α (TNF-α) and transforming growth factor-β (TGF-β), and the number of monocyte/macrophage cells that are the likely source of the cytokines were simultaneously measured in plastic-embedded bone marrow (BM) biopsy sections of 145 patients with myelodysplastic syndromes (MDS). TNF-α was correlated with TGF-β (P = .001) and with monocyte/macrophage cells (P = .003). Patients with excess blasts in their marrows had a higher TGF-β level (P = .01) and monocyte/macrophage number (P = .05). In a linear regression model, TGF-β emerged as the most significant biological difference between patients who have excess of blasts and those who do not (P = .01). We conclude that in addition to TNF-α, TGF-β also plays a significant role in the initiation and pathogenesis of MDS, and that a more precise definition of its role will likely identify better preventive and therapeutic strategies.