A technique for cancer treatment using focused microwave arrays to induce hyperthermia is developed. The system as initially proposed consists of orthogonal linear-phased arrays whose focusing properties are allied to the weakness of malignant cells to excessive heat. A two-dimensional computer simulation of the power dissipation inside a cylindrical body, with uniform permittivity and attenuation, shows that the array system could provide sufficient localised heating within the attenuating medium. The feasibility of operating without a matching dielectric is examined, but the results so far obtained are inferior in some important respects to those obtained with a matching dielectric.