We have devised a transit dose technique for fast neutron therapy treatment planning based on the16O(n,p)16N reaction in recirculating water, and have determined the effect of simulated bone and lung inhomogeneities in phantom. An effective threshold of 10.2 MeV in the16O(n,p) reaction is exploited to detect transmitted neutrons without the need for detector collimation. This system has been demonstrated with 14 MeV (d,T) neutrons and with cyclotron producedp(42)+Be neutrons.16N decays to the 6.13 MeV excited states of16O in 7.14 s, allowing for easy identification by NaI(Tl) and for rapid recirculation. The transmission of fast neutrons can thus be related to an effective thickness of soft tissue, providing a rapid and direct measure of the effects of inhomogeneities under actual treatment conditions, with the 10 MeV threshold providing a useful degree of insensitivity to multiply scattered neutrons. Equivalent thicknesses of compact bone and lung relative to water were found to be 1.4 and 0.34 respectively, closely resembling the effective thicknesses for Cobalt‐60 gamma rays.