He found the first documented evidence at the "Defense Agency Library" of Tokyo that the Imperial Japanese Army established and ran "comfort stations."
He discovered plenty of documentation and testimony to prove the existence of approximately 2,000 comfort women centers where as many as 200,000 Korean, Filipina, Taiwanese, Indonesian, Burmese, Dutch, Australian, and Japanese women, many of whom were teenagers and some as young as fourteen, were detained and forced to perform sexual activities with Japanese troops.
"[2] The publication of these documents led to admission statements by the Chiefs of the Cabinet: Secretary Koichi Kato on 12 January 1993 and Yōhei Kōno on 4 August 1993.
[3] In July 2004, Yoshimi and historian Yuki Tanaka proclaimed the discovery of documents at National Archives of Australia that demonstrated that cyanide gas was tested on Australian and Dutch prisoners in November 1944 on the Kai Islands.
[4] On 17 April 2007, Yoshimi and fellow historian Hirofumi Hayashi told a news conference that they had discovered documents at the archives of the Tokyo Tribunal that demonstrate that Tokkeitai members coerced or otherwise forced women from Indonesia, Indochina, and China into sexual slavery.