Oblique incidence of an electron beam can alter the central axis depth dose. The incident beam can be considered to be an integration of many pencil beams or slit beams. Depending on the depth in the phantom, neighboring pencil beams may have a greater or lesser contribution to the dose at a point on the central axis compared to the contribution under normal incidence. The effect was studied experimentally and theoretically. For 6- and 9-MeV electron beams, oblique incidence produces an increased dose at shallow depths and a decreased dose at normal treatment [patient] depths.