Following the German surrender in May 1945 Kahr, spent several months in American military detention. Due to his reputable standing with many former prisoners of Mittelbau-Dora, he was not among those indicted for war crimes by US occupation authorities during the 1947 Dora Trial, and appeared as a key witness for the prosecution in that proceeding as well as at the Pohl Trial the following year. After his release from US custody, Kahr returned to Graz where he resumed the practice of medicine.