In the conformation technique a target volume is irradiated uniformly as in conventional radiations, whereas surrounding tissue and organs are nonuniformly irradiated. Clinical data concerning radiation injuries that accumulate with conventional radiation are not applicable there without appropriate compensation. Recently a putative solution of this problem was proposed by Lyman using dose-volume histograms. This histogram reduction method reduces a given dose-volume histogram of an organ to a single step which corresponds to the equivalent complication probability by interpolation. As a result it converts nonuniform radiation into a unique dose to the whole organ which has the equivalent likelihood of radiation injury. This method is based on low LET radiation with conventional fractionation schedules, so when itis applied to high LET radiation such as negative pion treatment a high LET dose should be converted to an equivalent photon dose using an appropriate value of RBE. In the present study the histogram reduction method was applied to actual patients treated by the negative pion conformation technique at the Paul Scherrer Institute. Out of 90 cases with pelvic tumors that were evaluated 16 developed grade III-IV bladder injury, and 7 developed grade III-IV rectal injury. The 90 cases were divided into roughly equal groups according to the equivalent doses to the entire bladder and rectum. Observed complication rates and equivalent doses to the full organs in these groups could be represented by a sigmoid dose-effect relation. When RBE from a pion dose to a photon dose is assumed to be 2.1 for bladder injury, the observed rates of complications of the bladder fit best to the theoretical complication curve. When the RBE value was 2.3, the rates of rectal injury fit the theoretical curve best. These values are close to the conversion factor of 2.0 which is used in clinical practice at PSI. This agreement suggests the clinical feasibility of the histogram reduction method in conformation radiotherapy.