Q fever is a worldwide zoonosis which is caused by Coxiella burnetii and presents as both acute or chronic cases. The disease can be transmitted from animal reservoirs to humans by the inhalation of infected aerosols. The authors investigated the epidemiology of Q fever in the Bouches-du-Rhône district of southern France. The study area was centered around the small town of Martigues near the cities of Marseille and Aix-en-Provence, where the incidence of the disease seemed higher than in neighboring areas. Epidemiologic data included sheep breeding and wind. Between 1990 and 1995, Q fever was diagnosed in 289 patients, leading to an incidence rate of 35.4 per 100,000 in the study area (range: 6-132), compared with 6.6 in the area of Marseille, and 11.4 in the area of Aix-en-Provence. There was a graphical and statistical relation between the sheep densities, the incidence of the disease, and the strong, local wind known as the Mistral, which blows from the northwest. Although Coxiella burnetii transmission is multifactorial, we may speculate that the high endemicity in the study area is related to a contamination by aerosols because the Mistral blows through the local steppe where 70,000 sheep are bred. This public health problem requires further studies in order to confirm this hypothesis, and to identify more individual and preventable risk factors.