Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a disorder that has an elevated frequency in Saguenay-Lac-St-Jean (SLSJ) and Charlevoix, two geographically isolated regions in the past of northeastern Quebec. The incidence at birth and the carrier rate in SLSJ were estimated at 1/1,932 liveborn infants and 1/22 inhabitants, respectively, for the period 1941-1985. The mean inbreeding coefficient was twice higher and the mean kinship coefficient 3 times higher among the ARSACS families than among control families. In the SLSJ region, the birth places of the ARSACS individuals and their parents did not show a clustered distribution. The genealogical reconstruction suggests that the high incidence of ARSACS in SLSJ and Charlevoix is likely to be the result of a founder effect. Because the disease is apparently unknown elsewhere in the world and a high proportion of French Canadians presently living in eastern Quebec have ancestors coming from Perche, a small region in France, it also suggests that a unique mutation accounts for most, if not all, of the ARSACS cases known in these regions.