Epidemic psychogenic illness or mass hysteria occurred in a telephone operators' building following a pronouncement by an operator of toxic fume exposure. Two outbreaks occurred within two days, resulting in mass evacuation and closure of the building. Extensive environmental studies failed to identify a causative substance. Eighty-one employees were treated in emergency medical facilities for clinical and laboratory findings compatible with anxiety and hyperventilation. Characteristics of this work group were similar to those described in other epidemics, suggesting a pattern for susceptibility. Failure of early recognition and overreaction by supervisors and various outside groups and agencies contributed to escalation of the epidemic.