In 2004 and 2005, staff at the town hall in Villejuif, Val-de-Marne, experienced an outbreak of unexplained irritative symptoms that caused widespread anxiety, was widely reported in the media, and led to the temporary closure of the premises. The investigation conducted in February 2005 focused on describing the health events by expanding the case search to the surrounding population, understanding the context in which they occurred, and exploring the hypothesis of environmental exposure. The outbreak affected 17% of staff in 2004 and 30% in 2005, with signs of irritation of the mucous membranes and skin, and more rarely general symptoms (fatigue, headaches); none of these were serious, but the outbreak had a strong collective dimension, and symptoms disappeared upon leaving the premises. It did not affect the surrounding population. Testing for toxic substances in the outdoor environment and within the premises was negative; however, the indoor air was stagnant. Symptoms appeared in both years within the same department, which was characterized by difficult working conditions. Investigators diagnosed "sick building syndrome," and recommendations for managing the incident were made to city hall officials and staff. The speed of the investigation, close coordination among the various parties involved, and employee engagement were factors contributing to the effective management of the incident. (R.A.)