Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a hematopoietic growth factor and a proinflammatory cytokine. While GM-CSF is lacking in normal brain tissue, it is expressed under pathological conditions and correlates with the presence of dendritic cells (DC). However, the role of GM-CSF for the onset of immune responses in the brain is still unclear. To analyze the role of GM-CSF for the induction and functional activity of immune cells in the brain, we performed chronic intracerebroventricular administration of GM-CSF to the brains of adult mice. After GM-CSF administration, intracerebral leukocytes (ICL) were characterized by means of flow cytometry, immunohistochemistry, and an ex vivo functional assay. GM-CSF treatment significantly increased the number of leukocytes expressing high levels of CD45, indicative of peripheral, blood-derived cells. The infiltrating cells were preferentially DC of the myeloid lineage (CD45(high) CD11c+ CD11b+) with an activated phenotype characterized by upregulated expression of MHCII and the costimulatory ligand CD80. Furthermore, DC from GM-CSF treated mice were fully competent to activate naive allogeneic T cells in a mixed leukocyte reaction. In contrast, intracerebroventricular IFN-gamma administration stimulated MHCII expression on cells resembling resident microglia, but did not induce comparable presence of DC. Taken together, intracerebroventricular GM-CSF treatment results in high numbers of DC in the brain. Moreover, these GM-CSF-induced DC display an activated phenotype and exhibit the capacity to act as fully competent DC even without a further inflammatory stimulus.