Autologous activated CD4(+) T lymphocytes (CD4(+) /CCR5(+) double-positive cells) that derived from BMNCs of patients with low and intermediate-1 risk myelodysplastic syndrome were depleted or added to in vitro cultures. The BMNCs depleted of CD4(+) /CCR5(+) T cells exhibited significantly increased numbers of colony-forming units (CFUs). Conversely, the bone marrow mononuclear cells cultures with a fourfold augmentation of CD4(+) /CCR5(+) T lymphocytes exhibited no colonies in cultures in vitro. The apoptotic index (AI) of colony cells was decreased compared with that of preculture counterparts. After depletion of CD4(+) /CCR5(+) in vitro cultures, the clonal cells increased in patients with chromosome 5q- or 20q- abnormalities but remained unchanged in patients with trisomy 8. In addition, after removal of CD4(+) /CCR5(+) T cells, the number of CFUs was increased in those patients with a higher number of BM Th1 (CD4(+) / IFN-γ(+) ) cells, hypocellularity, or bearing the DR15 allele. We concluded that the selective removal of autologous activated CD4(+) T cells can increase the generation of CFUs. However, whether the increased CFUs consisted of cells derived from residual normal hemopoiesis or clonal hemopoiesis remains unknown.