An implantable helical coil microwave antenna has been developed for improved localization and control of interstitial hyperthermia for deep-seated tumours. A helical coil structure was employed as an extension of the inner conductor at the terminal portion of a miniature semi-rigid coaxial cable. The antennas were constructed with three different connection configurations of the helical coil to the feedline, and with several coil turn densities during the optimization of heating characteristics. In order to compare relative antenna heating performance, a set of quantitative parameters was introduced. Power deposition profiles of 2450 MHz helical coil antennas were studied in both phantom models and muscle tissue in vivo, and compared to those of commonly used dipole antennas. Optimal antenna performance was obtained with a 10-turn per 1 cm helical coil connected to the inner conductor at the tip and separated from the outer conductor by a 0.1 cm gap (HCS-10). These antennas produced a well-localized heating pattern with a sharp falloff of temperature in both directions axially from the coil element. For half-wavelength insertion depths, the effective heating length (50 per cent of maximum SAR) of HCS-10 antennas matched that of standard dipole antennas, but was shifted down towards the tip. For shorter and deeper antenna insertion depths the HCS-10 heating pattern remained similarly localized to the region surrounding the helical coil with minimal cold zone at the tip. In contrast, the dipole antenna heating pattern changed significantly depending on insertion depth, with an unavoidable 0.2-0.7 cm cold region at the antenna tip and elevated surface temperatures for short insertion depths.