The depth of dose maximum,dmax, of megavoltage x‐ray beams was studied as a function of beam energy and field size for 6‐, 10‐, and 18‐MV x‐ray beams and field sizes ranging from 1×1 to 30×30 cm2. For a given beam energy,dmaxincreases rapidly with increasing field size at small fields, reaches a maximum around 5×5 cm2and then gradually decreases with increasing field size for large fields. Monte Carlo simulations combined with measurements verified that the effect observed at small field sizes is caused by in‐phantom scatter, while at large fields the effect is due to scatter contamination of the primary beam from the linac head. A comparison between thedmaxbehavior of flattened beams to that of unflattened beams indicates that thedmaxdecrease at large fields for flattened beams is caused mainly by contamination electrons which are produced in the flattening filter and further scattered by collimator jaws and air.