A Monte Carlo electron-photon transport code was developed in order to determine the effects of static, longitudinal, magnetic fields on dose distributions produced by high-energy electron beams, and to optimize the design of a superconducting magnet system. As a result of these simulations, a 20-cm-i.d., 30-cm-o.d., 15-cm-tall, single-coil, magnet system was designed that could be incorporated into a mobile treatment table for use with a standard radiation therapy accelerator. Operating at a current density of 18 kA/cm2, the magnet would produce field strengths of 1-4 T in the phantom and 0.01 T at the accelerator exit window. Magnetically enhanced dose distributions, calculated for 20- and 30-MeV electron beams, show a pronounced Bragg peak, steeper gradients to the sides and rear, and a roughly fourfold increase in the peak dose to entrance dose ratios relative to those similarly calculated without a magnetic field. These magnetically enhanced dose distributions have the potential for sparing intervening tissue when high-energy electrons are used for the treatment of deep-seated tumors.