The presence of neutrophils in tumors has traditionally been considered to be indicative of a failed immune response against cancers. However, there is now evidence showing that neutrophils can promote tumor growth, and increasingly, the data support an active role for neutrophils in tumor progression to distant metastasis. Neutrophils have been implicated in promoting metastasis in cancer patients, where neutrophil numbers and neutrophil-related factors and functions have been associated with progressive disease. Nevertheless, the role of neutrophils in tumors, both at the primary and secondary sites, remains controversial, with some studies reporting their anti-tumor functions. This review will focus on the data demonstrating a role for neutrophils in both tumor growth and metastasis and will attempt to clarify the discrepancies in the literature.

Evidence is mounting for a role for neutrophils in breast cancer progression to metastasis. However, the role of G‐CSF in neutrophil biology in a cancer setting remains to be defined. Herein we discuss the most recent clinical and experimental evidence for neutrophils and G‐CSF in the promotion of metastasis, demonstrating a potential mechanistic link between them. Understanding this link is imperative both for the development of diagnostic tests and for therapies targeting neutrophils to improve the treatment of breast cancer patients with or at risk of developing metastatic disease. As a high neutrophil‐to‐lymphocyte ratio in patients predicts poor outcome, while mild neutropenia predicts an improved outcome, we urge caution in the use of G‐CSF in neutrophil recovery following chemotherapy as there is increasing evidence in preclinical models that G‐CSF can promote metastasis.