We have previously shown that the levels of hematopoietic progenitors in long-term marrow cultures (LTMC) from patients with aplastic anemia (AA) are drastically reduced, as compared to normal LTMC. We have also reported that when LTMC from AA patients are supplemented with recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) there is an increase in colony-forming cell (CFC) levels. However, such a stimulation is only transient and it is followed by an inhibition in CFC growth. Based on these observations, in the present study we have tested the hypothesis that the levels of tumor necrosis factor-α (TNF-α), an inhibitor of hematopoiesis, are increased in AA LTMC and that such levels are further increased after rhGM-CSF has been added to the cultures for several weeks. Accordingly, we have determined the levels of TNF-α in the supernatant of LTMC established from normal (n = 8) and AA (n = 6) bone marrow and in AA LTMC supplemented with rhGM-CSF (n = 6). At the time of culture initiation, TNF-α levels were below detection in all the samples analyzed. After 5 weeks of culture, TNF-α levels in normal LTMC were very low, with a median of 7.3 pg/mL. In contrast, AA LTMC contained higher levels of TNF-α (median of 49.6 pg/mL). In keeping with our hypothesis, addition of rhGM-CSF to AA LTMC resulted in a significant further increase of TNF-α levels (median of 135.4 pg/mL). Our results demonstrate an inverse correlation between reduced hematopoiesis in AA LTMC and increased levels of TNF-α in this culture system. Based on the results presented here, together with previous reports indicating that TNF-α is a potent inducer of apoptosis in hematopoietic progenitor cells, it seems reasonable to suggest that TNF-α is implicated in the pathophysiology of AA. Am. J. Hematol. 68:144–148, 2001. © 2001 Wiley-Liss, Inc.