It has been shown in several studies that the build-up in photon beams behind air cavities (such as in the head and neck) increases with energy. In this study this effect is investigated over a broad range of energies that have been used for treating head and neck tumours. The study addresses the question of whether an energy lower than 6 MV is desirable and is based on measurements and Monte Carlo (MC) simulations. In a PMMA phantom containing an air cavity (3 x 16 x 3 cm3 at 3 cm depth) an ionization chamber (Capintec PS-033) was used to measure the dose build-up behind the cavity for 4, 6 and 8 MV beam qualities for different field sizes (from 3 x 6 cm2 to 8 x 8 cm2). MC simulations were made using the EGSnrc code for the same geometry and energies as well as for Co-60. Measurements and MC simulations agree well when the fixed-separation plane-parallel chamber measurements have been corrected for the expected over-response in the build-up region. This work demonstrates that the build-up effect of 6 MV is 'closer' to the build-up effect of 8 MV than to that of 4 MV. This suggests that if the build-up effect is of concern when the target volume is in the vicinity of air cavities, 4 MV should be preferred over both 6 MV and 8 MV. This work also shows that the build-up effect for Co-60 is significantly smaller than that of 4 MV. Moreover, the build-up effect increases as the field size decreases. With the increasing use of IMRT (and radiosurgery), small fields are used more frequently making these issues even more relevant. This should be taken into consideration when choosing the accelerator energies for a radiotherapy department.