On Monday, November 6, 2000, at the Archet 2 Hospital in Nice, between 1:00 p.m. and 1:30 p.m., eight staff members from two adjacent departments—assisted reproductive technology and family planning—experienced "illnesses" characterized primarily by neurological-like manifestations associated with upper respiratory tract irritation, requiring hospitalization for some of them. In the days that followed, other staff members from various departments presented with similar symptoms, necessitating the establishment of a “targeted consultation” for staff beginning on Friday, November 10. In total, 635 staff members sought medical attention between November 6 and 26 for symptoms initially attributed to toxic fumes. This article presents the results of epidemiological investigations conducted to identify the cause(s) responsible for the clinical symptoms observed in staff working at Archet 2 during the period from October 26 to November 26, 2000. These results made it possible to rule out several of the hypotheses raised, notably a viral, bacterial, or fungal infectious cause. Despite a thorough investigation, no environmental cause could be identified. Two hypotheses can be put forward: the first concerns problems related to air conditioning and distribution, and the second concerns collective psychogenic symptoms.