A certain type of cell in the bone marrow can help promote tumor growth in mice with early stage lung cancer, a new study finds. Bone marrow derived cells have been shown to contribute to tumor progression, though many of the specific mechanisms through which this occurs remain unknown. Here, Camilla Engblom et al. found that mice with lung tumors had a significantly higher bone mass than their healthy counterparts, a pattern that was also seen in lung cancer patients. A closer look within the bone marrow of the tumor-bearing mice revealed a high number of osteocalcin-expressing (Ocn+) osteoblasts, cells that participate in bone formation. Intriguingly, deletion of Ocn+ cells suppressed tumor growth, indicating that these Ocn+ cells are functionally required for lung cancer progression. The authors next discovered that these Ocn+ cells stimulate production of a certain type of neutrophil that infiltrates the lung tumors. These neutrophils exhibited increased expression of genes associated with tumor-promoting processes, including angiogenesis, suppression of T cell responses, and tumor cell proliferation and growth, the authors report. They note that, in a small study of patients with lung cancer, the same neutrophil signature correlated with worse patient survival. This research is highlighted in a Perspective by Haiying Zhang and David Lyden.