pH-sensitive drug-delivering magnetic nanoparticles (DMNPs) have been developed as theragnostic nanocarriers that release doxorubicin (DOX) under acidic conditions within cancer cells. The drug release kinetics of the DMNPs, combined with the nanoparticle accumulation pattern in tumor cells, can be monitored by magnetic resonance imaging (MRI) to guide decisions regarding the optimization of drug dosing schedules.