Proceedings from a symposium on the diagnosis and amelioration of industry related mass psychogenic illness (MPI) is presented. Outbreaks of unusual behaviors, sickness and hysteria in manufacturing and assembly facilities and workplaces in the United States, and other countries including England and Singapore are reviewed. Outbreaks of MPI investigated by NIOSH also are included. Social comparisons, complexities, cautions and concepts in research on MPI are discussed. Stress, illness related behaviors, somatization, triggering events, theoretic and psychoanalytic approaches to the study of MPI are described with reference to concepts of convergence, contagion, emergent norms, core variable, sex difference, and coping with stress. Factors that affected symptoms including hysterical contagion, quality of the working environment, mechanization, automation, shift work, extra large enterprises, anonymity of the workers. Workers and management relations are discussed. The effect of MPI on rapid industrialization in developing countries in described and MPI and chemically induced hypersusceptibility are compared.