The presence of tumor-associated macrophages (TAMs) correlates with breast cancer progression and metastatic spread. Metastasis-associated macrophages (MAMs) are also recruited to distant sites, where they support metastatic growth. In this study, we demonstrate that in vivo fluorine-19 (19 F)-based MRI cell tracking can evaluate the density and distribution of macrophages within murine breast cancer tumors and associated metastases.