“Mass hysteria” is an epidemic diagnostic term used to characterize unexplained outbreaks of syncope among women. A syncope outbreak among women in a meeting in a rural area of Michigan prompted an intense investigation for etiology. Low levels of ozone, carbon monoxide, and pentane were found associated with the outbreak. These levels were too low by themselves to explain the symptoms. A sociometric scale of intensity of illness was devised and found to be highly correlated (r=−0.94) with the weight of those who fainted. It is argued that mass hysteria may be the result of interactions of low levels of toxicants and may not be a result of the “hysterical” behavior in women at all.