A total of 413 school students were taken ill with symptoms from an alleged poisonous gas affecting two different geographic locations at the same time and without influencing the residents in the neighbourhood. Symptoms were mainly subjective and almost all were discharged from hospital the same day. Vigorous endeavours by authorities had failed to discover the offending agent. By the use of epidemiological methods, the present study attempted to investigate the nature of and the significant contributing factors related to the episode. The psychological nature was confirmed both by demonstration of positive characteristics as well as by the exclusion of organicity. There was no direct evidence that teachers, headmasters, parents or the mass media had significant influence on the aetiology of the epidemic, albeit their influence could still be an indirect one. Regarding symptom formation, there was evidence that peer influences were more important than an individual's own previous experiences.