Prior to joining Aum, Hayashi was a senior medical doctor with "an active 'front-line' track record" at the Japanese Ministry of Science and Technology. Himself the son of a doctor, Hayashi graduated from Keio University, one of Tokyo's top universities. He was a heart and artery specialist at Keio Hospital, which he left to become head of Circulatory Medicine at the National Sanitorium Hospital in Tokai, Ibaraki. Somewhere along the line Hayashi became disillusioned and decided to look beyond conventional medicine. He found AUM during his search. In 1990 Hayashi resigned his job and left his family to join Aum and become a renunciate. He was believed to be one of Shoko Asahara's favorites and was appointed the group's "Minister of Health" with responsibilities encompassing a broad range of activities from development of a balanced "Aum diet" and supervision over "Aum hospital" to experimentations with the illegal psychedelic drugLSD
During the Tokyo sarin gas attack, Ikuo Hayashi and Tomomitsu Niimi were involved in delivering the sarin containers on the Chiyoda line as a part of the Tokyo sarin attacks. Niimi served as the driver. Hayashi, wearing a surgical mask of the type commonly worn by Japanese during cold and flu season, boarded the southwestbound 7:48 a.m. Chiyoda line train number A725K on the first car. He was about to puncture the sarin packs but he had second thoughts due to seeing dozens of commuters and fearing that releasing the sarin would endanger their lives, he decided to do it at the next stop. Hayashi punctured only one bag of sarin at Shin-Ochanomizu Station in the central business district. Two transit authorities in the subway were killed by the sarin, including the assistant station-master, and 231 suffered serious injuries. Both perpetrators did not attempt to hide and returned to Aum facilities. Hayashi later reported to the Japan police investigators about who were the perpetrators of the attack and the Aum activities post-attack. A huge raid of Aum facilities and arrest of Aum members were conducted.
In court, the two reacted differently. While Niimi refused to cooperate with prosecution saying that the "four meditative concentrations" he experienced under guidance of Asahara prevent him from testifying against his Guru, Hayashi accepted responsibility in the attack and testified in court. He was quoted as saying that he regrets that "while he became a doctor to help, he used his skills to inflict sufferings". He apologized to the victims and said that he views the death penalty as an appropriate punishment for his deeds. Niimi, citing different reasons, also said that he would "gladly" accept the capital punishment. Hayashi was given a life sentence at the end of the trial while Niimi was sentenced to death.
