Brain metastases are difficult to treat and mostly develop late during progressive metastatic disease. Patients at risk would benefit from the development of prevention and improved treatments. This requires knowledge of the initial events that lead to brain metastasis. The present study reveals cellular events during the initiation of brain metastasis by breast cancer cells and documents the earliest host responses to incoming cancer cells after carotid artery injection in immunodeficient and immunocompetent mouse models. Our findings capture and characterize heterogeneous astrocytic and microglial reactions to the arrest and extravasation of cancer cells in the brain, showing immediate and drastic changes in the brain microenvironment on arrival of individual cancer cells. We identified reactive astrocytes as the most active host cell population that immediately localizes to individual invading tumor cells and continuously associates with growing metastatic lesions. Up-regulation of matrix metalloproteinase-9 associated with astrocyte activation in the immediate vicinity of extravasating cancer cells might support their progression. Early involvement of different host cell types indicates environmental clues that might codetermine whether a single cancer cell progresses to macrometastasis or remains dormant. Thus, information on the initial interplay between brain homing tumor cells and reactive host cells may help develop strategies for prevention and treatment of symptomatic breast cancer brain metastases.