Purpose To investigate the relationship between changes in vascularity and metabolic activity measured by dynamic contrast‐enhanced magnetic resonance imaging (DCE‐MRI) and dynamic 18F‐FDG‐positron emission tomography (PET) in breast tumors undergoing neoadjuvant chemotherapy. Materials and Methods PET and MRI examinations were performed in 14 patients with locally advanced breast cancer (LABC) before and after chemotherapy. Dynamic 18F‐FDG PET measures included 18F‐FDG transport rate constant from blood to tissue (K1) and metabolism flux constant (Ki). DCE‐MRI measures included initial peak enhancement (PE), signal enhancement ratio (SER), and tumor volume. Spearman rank‐order correlations were assessed between changes in PET and MRI parameters, and measures were compared between patients with and without pathologic complete response (pCR) by Mann–Whitney U‐test. Results Changes in glucose delivery (PET K1) were closely correlated with changes in tumor vascularity as reflected by DCE‐MRI SER (r = 0.83, P < 0.001). Metabolic changes in PET Ki showed moderate correlations with vascularity changes as reflected by SER (r = 0.71) and PE (r = 0.76), and correlated closely with MRI tumor volume (r = 0.79, P < 0.001). Decreases in K1, Ki, SER, and PE were greater for patients with pCR compared to those with residual disease (P < 0.05). Conclusion Dynamic 18F‐FDG PET and DCE‐MRI tumor measures of tumor metabolism, vascularity, and volume were well correlated for assessing LABC response to neoadjuvant chemotherapy and significantly discriminated pathologic complete responders. Further work is necessary to assess the value of combined PET and MRI for evaluating tumor pharmacodynamics in response to novel therapy. J. Magn. Reson. Imaging 2010;32:1124–1131. © 2010 Wiley‐Liss, Inc.