Allogeneic bone marrow transplant recipients maintain normal peripheral blood counts long term, suggesting durable support from engrafted stem cells. In order to investigate late hemopoietic reconstitution at the level of committed and early progenitors (LTC-IC), we studied 64 long-term survivors at a median interval of 6 years (range: 2-20) after allogeneic bone marrow transplant. CFU-GM and BFU-E numbers did not differ from normal controls; CFU-GEMM were found to be significantly decreased (1.2 +/- 0.2/10(5) vs 3.1 +/- 0.4, P = 0.001). The most remarkable defect was however, the low frequency of LTC-IC (3.2 +/- 0.6/10(6) vs 54.2 +/- 9.3, P = 0.0001) that did not improve with time and did not correlate with phase of the disease, conditioning regimen, CMV infections or GVHD. Number of infused cells and CFU-GM content of marrow grafts did not seem to influence the number of LTC-IC. This study documents a significantly reduced number of early progenitors in BMT patients despite normal numbers of committed progenitors and normal peripheral blood counts. This finding may suggest a permanent reduction of the stem cell reservoir after allogeneic bone marrow transplantation.