For megavoltage x-ray beams, the percent depth dose increases considerably with field size in the buildup region, with a concomitant shift in the position of the maximum dose (dmax) to a shallower depth. Various authors disagree as to the cause of this effect. The radiation components contributing to absorbed dose in the buildup region of 10-MV photon field were analyzed as a function of field size by placing an electromagnet next to the Clinac 18 treatment head. The percent depth dose curves in the buildup region, obtained with a parallel plate chamber downstream from the magnetic field at 85 cm SSD, exhibited no dmax shift and a much reduced dependence on field size, in clear distinction with similar data taken with zero magnetic field. Confirmatory data were obtained at 100 and 120 cm SSD. These results clearly show that scattered electrons are the primary cause for the dmax shift and the dose increase in the buildup region with increasing field size.