The molecular basis for regulation of dendritic cell (DC) development and homeostasis remains unclear. Signal regulatory protein α (SIRPα), an immunoglobulin superfamily protein that is predominantly expressed in DCs, mediates cell-cell signaling by interacting with CD47, another immunoglobulin superfamily protein. We now show that the number of CD11c^high DCs (conventional DCs, or cDCs), in particular, that of CD8⁻CD4⁺ (CD4⁺) cDCs, is selectively reduced in secondary lymphoid tissues of mice expressing a mutant form of SIRPα that lacks the cytoplasmic region. We also found that SIRPα is required intrinsically within cDCs or DC precursors for the homeostasis of splenic CD4⁺ cDCs. Differentiation of bone marrow cells from SIRPα mutant mice into DCs induced by either macrophage-granulocyte colony-stimulating factor or Flt3 ligand in vitro was not impaired. Although the accumulation of the immediate precursors of cDCs in the spleen was also not impaired, the half-life of newly generated splenic CD4⁺ cDCs was markedly reduced in SIRPα mutant mice. Both hematopoietic and nonhematopoietic CD47 was found to be required for the homeostasis of CD4⁺ cDCs and CD8⁻CD4⁻(double negative) cDCs in the spleen. SIRPα as well as its ligand, CD47, are thus important for the homeostasis of CD4⁺ cDCs or double negative cDCs in lymphoid tissues.