Objective: This study was conducted to test the hypothesis that surgery induces changes at the expression level of genes implicated in metastasis, thus leading to accelerated postoperative metastatic tumor growth.Summary Background Data: Surgical resection of the primary tumor is a necessary and effective treatment for breast cancer patients. However, studies from both animals and humans have shown that surgery potentiates the growth of minimal residual neoplastic disease.Methods: Female BALB/c mice were inoculated with metastatic murine mammary adenocarcinoma 4T1-green fluorescent protein (GFP) cells in the mammary fat pad (3 x 10(5)/mouse), and divided into a surgery group (n = 12) in which the flank tumor was completely resected after 21 day growth and a control (no surgery) group (n = 12). Metastatic tumor burden was assessed by both macroscopic metastatic nodule count and clonogenic assay. Mitotic and apoptotic indices were established using a combination of hematoxylin-eosin histology and Ki-67 immunohistochemistry. Green fluorescent protein (GFP) expressing tumor cells were isolated using FACS sorting, and RNA was extracted. The RT2 Profiler PCR Array mouse Cancer Pathway Finder was used to determine and compare the mRNA levels of 84 genes involved in metastasis in both groups.Results: Excision of the primary tumor was associated with increased systemic metastatic burden (P = 0.001). Postoperative metastases exhibited increased proliferation (P = 0.001), but no reduction in apoptosis. The quantitative real-time polymerase chain reaction array data indicate that surgery significantly upregulated the expression of Itgb3, Egfr, Hgf, Igf1, Pdgfb, Tnf alpha, Vegfa, Vegfc, and MMP9 genes, and led to the down regulation of Cdkn2a, Cdh1, and Syk genes. Increased expression of ITGB3 and MMP9 was further confirmed at the protein level by Western blot.Conclusions: Removal of the primary tumor led to a progressive phenotype of lung metastases that exhibited upregulation of genes involved in adhesion, invasion, and angiogenesis.