To address the role of transforming growth factor (TGF) β in the progression of established tumors while avoiding the confounding inhibitory effects of TGF-β on early transformation, we generated doxycycline (DOX)-inducible triple transgenic mice in which active TGF-β1 expression could be conditionally regulated in mouse mammary tumor cells transformed by the polyomavirus middle T antigen. DOX-mediated induction of TGF-β1 for as little as 2 weeks increased lung metastases >10-fold without a detectable effect on primary tumor cell proliferation or tumor size. DOX-induced active TGF-β1 protein and nuclear Smad2 were restricted to cancer cells, suggesting a causal association between autocrine TGF-β and increased metastases. Antisense-mediated inhibition of TGF-β1 in polyomavirus middle T antigen-expressing tumor cells also reduced basal cell motility, survival, anchorage-independent growth, tumorigenicity, and metastases. Therefore, induction and/or activation of TGF-β in hosts with established TGF-β-responsive cancers can rapidly accelerate metastatic progression.