Magnetic nanoparticles are being developed for a wide range of biomedical applications. In particular, hyperthermia involves heating the magnetic nanoparticles through exposure to an alternating magnetic field. These materials offer the potential to selectively treat cancer by heating cancer tissue locally and at the cellular level. This may be a successful method if there are enough particles in a tumor possessing a sufficiently high specific absorption rate (SAR) to deposit heat quickly while minimizing thermal damage to surrounding tissue. High SAR magnetic nanoparticles have been developed and used in mouse models of cancer. The magnetic nanoparticles comprise iron oxide magnetic cores (mean core diameter of 50 nm) surrounded by a dextran layer shell for colloidal stability. In comparing two similar systems, the saturation magnetization is found to play a crucial role in determining the SAR, but is not the only factor of importance.(A difference in saturation magnetization of a factor of 1.5 yields a difference in SAR of a factor of 2.5 at 1080 Oe and 150 kHz.) Variations in the interactions due to differences in the dextran layer, as determined through neutron scattering, also play a role in the SAR. Once these nanoparticles are introduced into the tumor, their efficacy, with respect to tumor growth, is determined by the location of the nanoparticles within or near the tumor cells and the association of the nanoparticles with the delivered alternating magnetic field (AMF). This association (nanoparticle SAR and AMF) determines the amount of heat generated. In our setting, the heat generated and the time of heating (thermal dose) provides a tumor gross treatment response which correlates closely with that of conventional (non-nanoparticle) hyperthermia. This being said, it appears specific aspects of the nanoparticle hyperthermia cytopathology mechanism may be very different from that observed in conventional cancer treatment hyperthermia.