The pathogenesis of granulopoietic failure in systemic lupus erythematosus (SLE) was studied. In 16 Japanese women with SLE, a decreased number of granulocyte/monocyte progenitor cells (CFU-C) in the bone marrow was demonstrated, and the number of CFU-C correlated significantly with the peripheral blood granulocyte/monocyte count. The peripheral and bone marrow T lymphocytes suppressed the colony formation of autologous or allogeneic bone marrow CFU-C. These findings suggest that the decreased marrow CFU-C may be due to suppression by T lymphocytes, an event that may play an important role in the pathogenesis of granulopoietic failure in SLE.