Monodisperse cubic spinel iron oxide magnetic nanoparticles with variable sizes were prepd. following a multi-injection seeded-growth approach. As expected from such a known synthetic route, all samples were characterized by narrow size distributions, and showed excellent stability in both org. and aq. media without the presence of aggregates, thus becoming ideal candidates for the study of their hyperthermia performance. Specific Loss Power measurements indicated low heating powers for all samples without a max. for any specific size, contrary to what theory predicts. The magnetic study showed the formation of size-dependent nonsatd. magnetic regions, which enlarged with the particle size, evidencing a clear discrepancy between the crystal size and the effective magnetic vol. Strain map anal. of high resoln. transmission electron micrographs indicated the presence of highly strained crystal areas even if nanoparticles were monocryst. The origin of the crystal strain is strictly correlated with the seeded-growth synthetic procedure used for the prepn. of the nanoparticles, which turned out to alter their magnetic structure by creating antiphase boundaries. Considering the calcd. effective magnetic vols. and their magnetic dispersions in each sample, a reasonable agreement between hyperthermia expts. and theory was obtained.