We found a large deletion (more than 10 kilobases) in the gene for the low-density-lipoprotein receptor in 63 percent of French Canadians with heterozygous familial hypercholesterolemia. The deletion also occurred in homozygous form in four of seven French Canadian homozygotes. The deletion removes the promoter and first exon of the gene, and it abolishes the production of messenger RNA for the low-density-lipoprotein receptor. The high frequency of this mutation is presumably related to a founder effect among the 8000 ancestors of present-day French Canadians, who have had relatively little cross-breeding with groups of other national origins. This deletion has not been observed in any other ethnic group. It can be detected by analysis of genomic DNA from blood leukocytes, thus allowing direct diagnosis of familial hypercholesterolemia in a majority of affected French Canadians.