The passage of Storm Klaus through southwestern France in January 2009 left approximately 1.7 million customers without power and led to an unprecedented increase in carbon monoxide (CO) poisoning cases. Daily targeted monitoring made it possible to document these household incidents and the circumstances surrounding them, as well as to identify specific preventive measures. A case was defined as an accidental domestic CO poisoning occurring between January 24 and 30, 2009, in the Aquitaine, Midi-Pyrénées, and Languedoc-Roussillon regions. Data were collected from the medical-environmental questionnaires of the CO poisoning surveillance system. The number of poisonings and their characteristics were compared with data collected during the winters of 2006 to 2008. In the wake of the storm, 109 episodes of CO poisoning (4 expected) were reported, involving 295 people (14 expected), including 4 deaths. Rural communities, single-family homes, and those equipped with electric heating were the most affected. The majority of poisonings (74%) were linked to the use of generators as a temporary source of electricity. This study demonstrated an increase in the number of cases as well as specific circumstances surrounding the occurrence of CO poisoning related to power outages. This outbreak led to the subsequent development of a prevention plan incorporating informational materials targeting the general public, initiatives aimed at generator rental and sales companies, and the inclusion of prevention messages in Météo-France weather advisories. (R.A.)