This article presents a social-psychological theory to explain collective behavior involving unusual somatic reactions. The authors term this social phenomenon a "collective anxiety attack," to distinguish it from the psychiatric label of "mass psychogenic illness." First, the article contrasts fundamental assumptions of a psychiatric-medical interpretation with a social-psychological interpretation. Next, the article illustrates the difference using a reinterpretation of and new data about the classic case of the "mad gasser of Mattoon." Then, the authors present a social-psychological theory which suggests that collective anxiety is induced by a shared belief in a threat rumor. Once a belief in an imminent threat spreads widely enough to create a consensual definition of the situation, the belief intensifies fear and distorts individual perception. Belief in the threat rumor creates psychological stress and expectancy effects that shape an individual's somatic reactions, so that expectations of feeling sick result in symptoms of sickness. In conclusion, the article suggests a program for research and ways of managing collective anxiety attacks.Or in the night, imagining some fear, How easy is a bush suppos'd a bear! William Shakespeare, "A Midsummer Night's Dream," act 5, scene 1, lines [21][22] This study presents a new social-psychological theory of collective behavior involving unusual somatic reactions. The theory emphasizes the role of normal sociocultural and social-psychological processes, rather than psychopathology. The authors apply this new theory to the much-cited episode of the "mad gasser" of Mattoon (vide Johnson 1945), which involved strange somatic reactions to imaginary gas attacks. In conclusion, the authors offer applications of the theory for research and management of outbreaks of widespread imaginary illnesses.