In early October 2017, approximately 30 students from a middle school in the Pyrénées-Orientales were treated for suspected carbon monoxide (CO) poisoning. Since initial environmental tests did not detect the presence of CO in the school, an epidemiological investigation involving face-to-face questionnaires with the affected students was conducted in addition to the environmental investigation to identify the cause of this incident. A variety of symptoms were reported by 35 students, representing an incidence rate of approximately 6% among the entire student body. The data collected during the investigations ruled out the hypothesis of chemical or infectious poisoning and led to a diagnosis of a mass psychogenic illness with a strong emotional component. All the characteristics of this type of syndrome, widely described in the literature, were present: nonspecific symptoms (headaches, abdominal pain, etc.), a higher incidence rate among girls (9.9% vs. 1.3% among boys), and the presence of an anxiety-inducing trigger (an initial episode of illness followed by dramatic medical intervention). Recommendations aimed at preventing the risk of recurrence within the school were formulated.