Introduction Antiprotons as a new beam modality in radiotherapy are being investigated by the AD-4/ACE collaboration since 2002. A beam of antiprotons hitting a water phantom exhibit a similar depth-dose curve as that known from protons, except that the Bragg-peak is significantly pronounced due the resulting annihilation events occurring at the end of the antiproton particle tracks. It has so far been anticipated, that the radiobiology of antiproton beams is similar to that of protons in the entry region of the beam, but very different in the annihilation region, due to the expected high-LET components resulting from the annihilation. Though, the calculations of dose-averaged LET in the entry region may suggest that the RBE of antiprotons in the plateau region could significantly differ from unity. Materials and Methods Monte Carlo simulations using FLUKA were performed for calculating the entire particle spectrum of a beam of 126 MeV antiprotons hitting a water phantom. The beam was set to a square field and all scoring is done in a limited region along the central axis. Custom user routines were written for FLUKA in order to extract the fluence for each particle species as a function of energy per nucleon. Results In the plateau region of the simulated antiproton beam we observe a dose-average LET of about 4 keV/µm which is very different from the expected 0.6 keV/µm of an equivalent primary proton beam. Even though the fluence of secondaries is a magnitude less than the fluence of primary particles, the increased stopping power of the secondary causes an increase in the dose averaged LET. The maximum LET is recorded in the peak as 19 keV/µm. Discussion The dose-average LET in the plateau region may result in a RBE different from unity in the plateau region. RBE measurements of antiproton beam was carried out in October 2007 and preliminary estimations sustain the findings, though the Co-60 reference data for the RBE estimations are still to be performed.