The potential of negative pi mesons for radiation therapy has been informally suggested by many people, including Chaim Richman, now of the Los Alamos Scientific Laboratory. Fowler and Perkins (Ref. 1) were the first to make detailed calculations, and this work generated heightened interest in the use of negative pions for therapy. Their calculations showed that the strong nuclear inter-action experienced by pions when they come to rest could greatly increase the dose to the treatment volume. Figure 1 shows the depth-dose distribution expected from a 46 -MeV beam. The pions are captured by nuclei, primarily oxygen, nitrogen and carbon in tissue, which promptly break up into energetic but short-range charged fragments and neutrons. The total energy produced by breakup of the capturing nucleus is approximately 140 MeV; about 40 MeV goes into overcoming the binding energy of the capturing nucleus, about 70 MeV is carried off as kinetic energy of neutrons, a small amount as energetic nuclear gamma rays, and the rest (about 30 MeV) as kinetic energy of protons, alpha particles (helium nuclei) and heavier nuclear fragments (lithium and carbon nuclei). It is these high LET radiations (protons, alpha particles and heavier nuclei) that enhance the dose in the terminal region of the negative pion beam. Mesic X rays, nuclear gamma rays and neutrons are produced also but escape from the body without doing much damage either to the tumor or the surrounding normal tissue. Therefore, the depth-dose distribution from a pion beam does not drop off exponentially with depth (as with gamma rays, X rays and neutrons) but, rather, increases gradually until the stopping region is reached, where the high LET dose enhancement occurs. The finite range of pions in tissue also affords restriction of dose to normal tissue distal to the treatment volume to the muon and electron beam contamination.