A new 4 MeV linear accelerator, utilizing a compact standing waveguide accelerator whose x-ray beam is colinear with th e electron beam, is described. An output of 350 rads /rninute is possible with a sharp beam and a narrow penumbra. Isodose curves are discussed and transverse plots analyzed in detail; decrement lines for different field widths are plotted and their significance established. Also included are rotational aspects and clinical considerations concerning adjacent fields and the dose at depth in the region of varioussize gaps.