During the six years since the Aum Shinrikyo cult's nerve gas attack on the Tokyo subway system, the United States has undertaken a major effort to prevent and respond to terrorist acts involving nuclear, radiological, biological and chemical weapons. Given this high level of official activity, it is remarkable that Aum's protracted and costly, but ultimately unsuccessful, attempts to acquire and use mass-casualty biological agents has received scant analytical attention. Specifically, there has been relatively little focus on one critical question: Why did this apparently sophisticated and lavishly funded program ultimately fail? Aum's failure suggests that it may, in fact, be far more difficult to carry out a deadly bioterrorism attack than has sometimes been portrayed by government officials and the press. Despite its significant financial resources, dedicated personnel, motivation, and freedom from the scrutiny of the Japanese authorities, Aum was unable to achieve its objectives. The Aum case illustrates how acquiring virulent strains of biological agents can be a major hurdle for prospective bioterrorists. Cult-like terrorist organizations, the ones that appear to have the greatest interest in mass-casualty biological weapons, may be least suited to meet the complex demands associated with a bioweapon program. As the Aum example illustrates, a paranoid, fantasy-prone and sometimes violent atmosphere is not conducive to the sound scientific judgement needed to produce mass-casualty biological weapons.