In interstitial hyperthermia using ferromagnetic seeds, multi-filament seeds have gained interest because of a more effective power absorption than solid seeds. Palladium - nickel (PdNi) seeds composed of filaments with diameters in the range from 0.1 to 1.0 mm (maximally 90 filaments) have been investigated to find the conditions for optimal power absorption and temperature control. Magnetic and calorimetric experiments have shown that a decreasing filament radius results in a more effective power absorption. The power absorption approaches a common asymptote for high field intensities at all filament diameters. This asymptotic behaviour can be understood as a consequence of the approach of saturation magnetization of PdNi. The sharpness of the transition at the Curie temperature, which is a measure for the quality of temperature control, improves as the magnetic field strength increases, but it is limited by the asymptote of the power absorption. When the asymptote has been reached the quality of temperature regulation of a seed can only be improved by increasing the amount of PdNi, e.g. by increasing the number of filaments. Calculations of the power absorption, using the generally applied theory based on a linear relation between the magnetization of PdNi and the magnetic field strength, do not correspond quantitatively with experimental results for seeds having an induction number smaller than the `optimal value' of 2.5. For these seeds the measured heat production is larger than the calculated one.