Granulocyte-macrophage colony-stimulating factor (GM-CSF), also known as colony-stimulating factor 2 (CSF2), is a monomeric glycoprotein secreted by macrophages, T cells, mast cells, natural killer cells, endothelial cells and fibroblasts that functions as a cytokine. Unlike granulocyte colony-stimulating factor, which specifically promotes neutrophil proliferation and maturation, GM-CSF affects more cell types, especially macrophages and eosinophils. GM-CSF is found in high levels in joints with rheumatoid arthritis and blocking GM-CSF as a biological target may reduce the inflammation or damage. Some drugs (e.g. otilimab) are being developed to block GM-CSF. In critically ill patients GM-CSF has been trialled as a therapy for the immunosuppression of critical illness, and has shown promise restoring monocyte and neutrophil function, although the impact on patient outcomes is currently unclear and awaits larger studies. GM-CSF stimulates monocytes and macrophages to produce pro-inflammatory cytokines, including CCL17. Elevated GM-CSF has been shown to contribute to inflammation in inflammatory arthritis, osteoarthritis, colitis asthma, obesity, and COVID-19.