A 45-mev electron linear accelerator of the traveling wave type has been installed in this hospital for the production of high-energy external electron beams intended for therapeutic purposes. The first electron beam was obtained from the accelerator on March 6, 1954. However, physical measurements and proper evaluation of the performance of the machine could be carried out only after the magnet system had been designed, manufactured, and installed; this system consists of two electromagnets, one for analyzing and the other for focusing, and is used for controlling the energy, the direction, and the dimensions of the electron beam. The experimental results of certain measurements are summarized here. The design of the magnet system, the target-end equipment, and the methods that were used in these measurements will be described in a later contribution. The output energy of the accelerator can be continuously varied from a minimum of 6 mev to a maximum of 36 mev at the present time. The reasons that the actually obtained maximum energy is short of the originally designed and stipulated 45 mev lie mainly in the present klystron which applies the power for the acceleration and does not produce the expected 18 megawatt peak power. With a different klystron tube and better adjustment, an output energy closer to the stipulated figure is expected. The energy spectrum, measured with 1 per cent energy resolution at the target, was found to be of 15 per cent width between half-intensity points and with little dependence on the peak energy at which the measurements were made. The average target current2 obtained without appreciable beam loading is 10−2 micro-ampere for a 100-sq. cm. field. From the energy spectrum and the collimator dimension, the total output beam current3 of the accelerator was calculated to be about 1 micro-ampere. The target current can be varied by a factor of 104, the lower limit being set by the field emission and the upper limit dependent on the beam loading. The electron beam at the target has a rectangular cross-section, the dimensions of both sides being adjustable from 1 to 12 cm. The current distribution over the target area was found to be uniform, with a steep drop at the geometrical boundaries. The electron beam at the target has a controllable energy resolution of 2 per cent maximum and is slightly divergent, with an approximate 2· divergent angle. The target current drops to one half of its peak value when the frequency changes are one part in 3 × 104. Therefore, because of frequency instability of the magnetrons and changes of mechanical dimensions of the cavities of the accelerator caused by power dissipation, etc., occasional tuning of the magnetron frequency is required for long exposure. However, this presents no particular difficulties.