On May 21, 1979, an outbreak of illness spread swiftly among elementary school students in a Boston suburb. Of 224 boys and girls attending an assembly, 34 were hospitalized with severe dizziness, weakness, hyperventilation, headache, nausea, and abdominal pain. Sudden remission of symptoms, preponderance in girls, and failure of an extensive epidemiological investigation to detect an organic cause indicated mass hysteria. To test the hypothesis that previous loss influenced a child's vulnerability to current loss and predisposed that child to mass hysteria, we compared the incidence of family disruption in the hospitalized children with that in the nonhospitalized children. A significantly higher rate of parental divorce (P less than .00005) and death within the family (P less than .0005) occurred among the hospitalized children. These findings suggest a relationship between childhood loss and susceptibility to mass hysteria.