An important goal in cancer chemotherapy is to sensitively and quantitatively monitor the response of individual patients' tumors to successful, or unsuccessful, therapy so that regimens can be altered iteratively. Currently, tumor response is monitored by frank changes in tumor morphology, yet these markers take long to manifest and are not quantitative. Recent studies suggest that the apparent diffusion coefficient of water (ADCw), measured noninvasively with magnetic resonance imaging, is sensitively and reliably increased in response to successful CTx. In the present study, we investigate the combination chemotherapy response of human breast cancer tumor xenografts sensitive or resistant to Paclitaxel by monitoring changes in the ADCw. Our results indicate that there is a clear, substantial, and early increase in the ADCw after successful therapy in drug sensitive tumors and that there is no change in the ADCw in p-glycoprotein-positive tumors, which are resistant to Paclitaxel. The mechanism underlying these changes is unknown yet is consistent with apoptotic cell shrinkage and a concomitant increase in the extracellular water fraction.