The field-size dependence of the photon output of linear accelerators in air has been attributed in part to changes in the amount of radiation backscattered from the collimator jaws into the dose monitor chamber. This possible effect was investigated for a variety of accelerators with energies from 4 to 15 MV by measuring the monitor unit rate (MU/min) for different collimator openings. This measurement was made without dose rate feedback control, i.e., with constant electron beam current in the accelerator. The monitor unit rate was independent of collimator setting for all machines tested. Hence, it is concluded that backscattered radiation from the collimator jaws into the dose monitor chamber does not contribute to the variation of output with field size.