Tumor progression is a complex, multistep process involving accumulation of genetic aberrations and alterations in gene expression patterns leading to uncontrolled cell division, invasion into surrounding tissue and finally dissemination and metastasis. We have previously shown that the Arg/Abl2 non-receptor tyrosine kinase acts downstream of the EGF receptor and Src tyrosine kinases to promote invadopodium function in breast cancer cells, thereby promoting their invasiveness. However, whether and how Arg contributes to tumor development and dissemination in vivo has never been investigated. Using a mouse xenograft model, we show that knocking down Arg in breast cancer cells leads to increased tumor cell proliferation and significantly enlarged tumor size. Despite having larger tumors, the Arg-knockdown (Arg KD) tumor-bearing mice exhibit significant reductions in tumor cell invasion, intravasation into blood vessels and spontaneous metastasis to lungs. Interestingly, we found that proliferation-associated genes in the Ras-MAPK (mitogen-activated protein kinase) pathway are upregulated in Arg KD breast cancer cells, as is Ras-MAPK signaling, while invasion-associated genes are significantly downregulated. These data suggest that Arg promotes tumor cell invasion and dissemination, while simultaneously inhibiting tumor growth. We propose that Arg acts as a switch in metastatic cancer cells that governs the decision to 'grow or go' (divide or invade).