Patient neutron therapy is presently in progress at several institutions using cyclotrons which range in acceleration capabilities for deuterium ions from 16 to 50 MeV. The information developed over this range of energies allows comparison of the important clinical parameters and their variation with energy. Information is presented for a series of charged particle reactions which produce neutrons and the basic physics parameters of interest such as neutron yield, average and maximum neutron energy, and shielding considerations are discussed. For the reaction Be(d,n) the influence of charged particle energy on the above parameters is discussed and the reflection of this basic data in the measurements of clinical importance such as dose buildup in the first few millimeters (skin sparing), central axis depth dose, isodose distribution, and tumor control- complication rate relationships are considered. The conflicting criteria which must be considered in the design of any such facility for neutron therapy are reviewed, with the reader allowed to reach his own decision as to what the optimum design energy is.