Purpose: Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) measured the early vascular changes after administration of TRA-8, bevacizumab, or TRA-8 combined with bevacizumab in breast tumor xenografts. Procedures: Groups 1-4 of nude mice bearing human breast carcinoma were injected with phosphate-buffered saline, TRA-8, bevacizumab, and TRA-8 + bevacizumab on day 0, respectively. DCE-MRI was performed on days 0, 1, 2, and 3, and thereafter tumors were collected for terminal deoxynucleotidyl transferase-mediated dUT nick end labeling and CD31 staining. Results: DCE-MRI measured a significant K trans change within 3 days after TRA-8 therapy that correlated with tumor growth arrest, which was not shown with statistical significance by histopathology at these early time points posttreatment. The K trans changes followed quadratic polynomial curves. Conclusion: DCE-MRI detected significantly lower K trans levels in breast tumor xenografts following TRA-8 monotherapy or combined therapy with bevacizumab. © 2010 Academy of Molecular Imaging and Society for Molecular Imaging.