Fibrocytes recruited from the bone marrow are monocytic cells that differentiate to collagen-producing macrophage-like cells in lung tumors. Analysis of the fibrocyte-associated molecular interplay in the cancer niche provided evidence for a hitherto unrecognized crucial role of fibrocytes in promoting lung tumor growth and metastasis, linked with tumor-supportive phenotypic switches in the cancer niche. Local upregulation of the endothelin system in this niche was demonstrated as a critical mediator of these processes, with endothelin blockade being a potential therapeutic concept.