To obtain accurate information for absorbed dose calculations in water for kilovoltage x-rays, the photon spectrum, planar fluence and the angular distribution of the photons at the collimator exit of the x-ray unit have to be known. The only way to obtain this information is by Monte Carlo (MC) simulation. Compared with the situation for high-energy photons and electrons, where in recent years numerous papers have been devoted to MC modelling of complete clinical accelerator units, there is a lack of similar work for kV x-ray units. A reliable MC model for a kV x-ray unit would allow the output information to be used in a treatment planning system for regular and irregular treatment fields. Furthermore, with MC simulation, perturbation factors of dose-measuring devices, such as those specified in codes of practice, can be calculated. In this work, the MC code EGS4/BEAM was used to build realistic models of two complete x-ray units. The tungsten target, exit window, collimator, additional filtration and applicator were taken into account. For some aspects of the work, a comparison was made with the simulations from another MC code, MCNP4B. The contribution to the characteristic radiation from electron impact ionization and from the photoelectric effect of reabsorbed bremsstrahlung photons was studied. Calculated and measured photon fluence spectra in air and half-value layers for a Philips MCN410 tube were compared for several anode voltages and additional filtrations. Results from the two codes agreed well, and the agreement with measured spectra was found to be good for energies above 50 keV but rather less good below that energy. For a Siemens Stabilipan 2 Th300 x-ray tube, HVLs and dose distributions in water were compared with measurements for several clinical x-ray qualities. For most of the combinations of radiation qualities and applicators, good agreement was obtained, although there were also some cases where the agreement was not so good. Electron contamination and photon build-up at the water surface were studied using MC simulation. The influence of depth on the photon spectral distribution was investigated. Both EGS4/BEAM and MCNP4B, in their default versions, handle inadequately the production of characteristic x-rays. This was found to have only a minor influence on the calculated dosimetric quantities. Simulations with MCNP4B required the use of several variance reduction techniques in order to obtain results within reasonable calculation times.