Saguenay Lac-Saint-Jean (SLSJ) is a geographically isolated region of northeastern Quebec in which several autosomal recessive disorders have a high incidence. We calculated the inbreeding coefficients of 567 probands and compared them to 1701 matched control individuals. The mean inbreeding coefficient of the group containing all 567 probands was 2.73 times higher than that of the controls (0.001772 versus 0.00065). Thirteen percent (75/567) of the probands were inbred, but only 5% were born to matings between spouses related as second-degree cousins or closer. No marriage between uncle and niece and only two marriages between first-degree cousins were identified in the disorder group. These results strongly suggest that the high incidence of the autosomal recessive disorders in SLSJ is the result of a founder effect.