Heterozygous familial hypercholesterolemia (FH) is a relatively common autosomal dominant disorder, which is characterized by elevated plasma concentrations of low density lipoprotein (LDL) cholesterol and early coronary heart disease. FH results from mutations in the gene encoding the LDL receptor (LDLR). In Canada, there is a founder effect for LDLR mutations in FH among individuals of French descent, most of whom reside in the province of Quebec. However, the spectrum of mutations in other regions, specifically in the populous and predominantly English-speaking province of Ontario, has not been studied. We sequenced the coding regions, promoter and intron-exon boundaries of the LDLR gene in 60 Ontario FH subjects from a variety of ethnic backgrounds other than French Canadian. We found 25 LDLR mutations in 34 subjects. Eleven LDLR mutations were novel, including two in-frame deletions of a single amino acid (one each in exons 2 and 4), two larger deletions that shifted the reading frame (one each in exons 4 and 10), five missense mutations (C42R, A370T, T413M, L561P and E760D) and two splice acceptor mutations (one each in introns 3 and 8). The results indicate that FH is more genetically diverse in Ontario than in Quebec. The results are also consistent with findings from investigations of the LDLR in FH conducted in other countries, in which PCR-based, exonby-exon sequencing uncovers small mutations in about half of the subjects screened. The gap in molecular diagnosis suggests that lesions not found by this sequencing strategy, such as larger scale LDLR mutations that cannot be amplified, may underlie a substantial number of cases of FH. Alternatively, there might be genetic heterogeneity underlying the FH phenotype, with contributions from other single or multiple genes.