Mutations in the lipoprotein lipase (LPL) gene, leading to partial or total inactivation of the enzyme, result in a hereditary clinical syndrome called familial LPL deficiency. The French Canadian population, which is primarily and historically located in the province of Québec, has the highest worldwide frequency of LPL-deficient patients. We have analyzed the prevalence, spatial distribution, and genealogy in the Québec population of a LPL gene mutation, M-207 (P207L in conventional notation), which changes the amino acid proline to leucine in position 207 of the LPL protein and inactivates the enzyme. Our results show that M-207 is the most prevalent LPL gene mutation among French Canadians and accounts for the largest proportion of LPL-deficient patients in this population. Genealogical reconstruction of French Canadian LPL-deficient patients point to 16 founders of M-207, all of whom migrated to Québec in the early seventeenth century from the north-western part of France, especially from the region of Perche. Most of the carriers of M-207 are, at present, found in Charlevoix, Saguenay-Lac-St-Jean regions of eastern Québec. On the basis of the number of homozygote M-207 LPL-deficient patients so far identified, we estimate that there are at least 31,000 carriers of this mutation in the province of Québec. This constitutes a large pool of individuals at risk for atherosclerosis and other lipid-related diseases, since LPL deficiency is considered to be a significant contributing factor in the etiology and development of these diseases.