Abstract: Fast neutron therapy is significantly enhanced by irradiating target cells, either in vitro or in vivo, with fast neutrons in the presence of a boron neutron capture agent having a plurality, and preferably at least nine, of .sup.10 B atoms per molecule of the agent. In other aspects, tumor cells are treated in vivo by administering to a human or non-human animal a boron neutron capture agent having at least nine .sup.10 B atoms per molecule of the agent in an amount effective to associate the boron neutron capture agent with the tumor cells, and then irradiating the tumor cells with fast neutrons. Suitable boron neutron capture agents may be based on polyhedral borane anion derivatives, on derivatives that comprise two polyhedral borane anion cages linked together to form a structure comprising 20 boron atoms, on polyhedral carboranes such as compounds of the formulas closo--C.sub.2 B.sub.n-2 H.sub.n, closo--CB.sub.n-1 H.sub.n.sup.--, or nido--C.sub.2 B.sub.n-3 H.sub.n.sup.