An influence of a beam forming system on absorbed dose distributions for electron beams from linear accelerator is studied by a Monte Carlo method. The Monte Carlo method is used in arbitary, user-defined complex geometries. The following elements of the beam defining system are taken into account is presented Monte Carlo calculations: scattering foils used to flatten radiation fields, a collimator used to limit treatment field and to protect sorrounding healthy tissues, an electron applicator used to generate irradiated fields with irregular shapes and a water phantom simulating a patient. The SHOWME code, a modified version of the GEANT3 code, is used to look for characteristic features of absorbed dose distribution and their dependence on various elements of the beam defining system. The calculations are made for 22 MeV primary electron pencil beam incident perpendicularly on a water phantom.