Myotonic Dystrophy: I Socio-Economic and Home-Environment Characteristics of the Patients. Myotonic dystrophy is a dominantly inherited genetic disorder which, in medical literature, has been linked to peculiar socio-economical conditions. In order to document this assertion, a sociological study was conducted in the Saguenay-Lac-Saint-Jean region (Quebec), using a representative sample (N = 218) drawn from the myotonic dystrophy population. A study of the patients' places of residence was also carried out. The results clearly indicate that myotonic dystrophy patients exhibit a socio-economic profile associated with disadvantaged milieux: high unemployment, low income and limited schooling. Many of these patients must rely on social welfare. Two families out of five (42%) live beneath the poverty line. The wage-earning history of these persons is closely linked to their self-evaluation of their working capabilities. After the age of forty, a high proportion of these people are unable to hold down a paying job and claim they are unable to do so. Within a well-screened urban area, a study of the patients' places of residence indicates census tracts where myotonic dystrophy is concentrated (ecological niches). These social areas are close to the downtown area and display characteristics of disadvantaged milieux. Accordingly, the My D gene seems to be confined to a specific socio-economic and socio-geographical environment, through which the spread of the gene is channelled among the population.