The proton beam formation for therapy has been studied with Monte Carlo simulations. This numerical study was performed in conjunction with the measurements made in the horizontal beam line of the Midwest Proton Radiotherapy Institute (MPRI) in the USA. The simulations included necessary beam-modifying devices of the beam line, and were compared to the dose distributions measured in water and in air using an ionization chamber. Therapeutic beams were produced using a double-scattering method in the transverse planes and using a rotating modulator in the beam direction. The results showed that important physical parameters of a therapy beam such as lateral penumbra and source to axis distance (SAD) can be simulated to agree with experimental values within 5 %.