Thin silicon detectors have been used to measure high LET doses in patients being treated with negative pions. Measurements were made on the skin, in the oral cavity, and in the rectum of a series of patients and of certain selected patients. An analysis is given of the LET distribution of the particles observed in the thin, flat detector, and the high LET dose fraction is calculated from the event size spectrum. The data provide verification of the doses calculated from treatment planning. By timing the occurrence of the peak in the high LET dose distribution with respect to the position of the rangeshifter, one obtains the water-equivalent thickness of the bolus and tissues in front of the detector. This is the complementary measurement to the high LET dose measurement.