Background: Carbon monoxide (CO) is the leading cause of death from poisoning, but the true extent of this public health problem remains poorly understood at the national level. Given the difficulty in diagnosing poisoning, as well as the low sensitivity of the former national surveillance system, a reform of this system was undertaken by the Directorate General of Health (DGS) with the support of the French Institute for Public Health Surveillance (InVS), based on proposals from the French High Council for Public Health (CSHPF) developed in December 2002; a more rigorous definition of poisoning cases and a more detailed collection of information were recommended. Using the national database centralized by the InVS, the Interregional Epidemiology Unit (Cire) analyzed data from 2005 in the Île-de-France region. Population and methods: The circumstances of poisonings and their reporting (Alerte form), medical care (Intoxiqué form(s)), and environmental management (Source, Environmental Investigation Summary, and Work Report forms) were studied using five databases linked by case identification numbers. Poisoning cases were classified into seven types based on their location or mode of occurrence: home, workplace, public access facility (ERP), fire, intentional, moving vehicle, and unknown. Confirmed cases of poisoning and their clinical severity were defined in accordance with the recommendations of the CSHPF. Results: During 2005, 214 reports of poisoning were recorded in the Île-de-France region. These poisonings occurred predominantly in the home (69%), with firefighters and analytical laboratories reporting the highest number of cases (33% and 47%, respectively). Women and individuals in the young and middle age groups (<45 years) were more frequently affected. Clinical manifestations, primarily headaches, were generally mild (72% classified as severity levels 0 to 2). Among the 659 people involved in a poisoning case, 313 were hospitalized (48%). Thirty deaths were reported, 9 of which were confirmed by a medical investigation. Forty-two poisoned individuals were treated with hyperbaric oxygen therapy (HBOT). From an environmental perspective, more than 80% of cases occurring in the home underwent a detailed investigation, with the source of the poisoning very often identified (95%). Poisoning was more common in single-family homes. Hot water production systems, such as boilers, water heaters, and bath heaters, were often the cause (51% and 23%, respectively). Poisoning was often attributed to a combination of several abnormal circumstances: poor exhaust of combustion gases (53% of cases), appliance malfunction (52% of cases), or improper use of a makeshift auxiliary heating device (20% of cases). Inadequate ventilation in the home was an aggravating factor in 72% of investigations. Annual inspection of the installation could be confirmed for fewer than 30% of boilers, and annual sweeping of flue gas exhaust ducts for 17% of gas boilers. Conclusions: The results for 2005 suggest that the downward trend in the number of people poisoned observed over several years in the Île-de-France region is continuing. Hot water production systems, particularly boilers, are very often to blame, but the inappropriate use of makeshift heating devices is also cited. Poor ventilation in the home very often constitutes an aggravating factor, and failure to comply with regulations regarding the regular maintenance of appliances and flue gas exhaust systems appears to be common. Regarding data collection, several improvements are needed, particularly regarding information on the reporting source (the involvement of firefighters is likely underestimated) and the data necessary to estimate the number of deaths, as mortality is a fundamental indicator of the surveillance system. Furthermore, the lack of detailed clinical data made it impossible to assess the alignment of medical practices with recommendations regarding the indication for hyperbaric oxygen therapy. Finally, as part of an approach to prevent the risk of poisoning, informing and raising awareness among users is a priority, as is the involvement of construction professionals and general practitioners to encourage the reporting of high-risk situations.