A mutation in low density lipoprotein (LDL) receptor gene causes an autosomal codominant disorder namely familial hypercholesterolemia (FH). Mutations in the LDL receptor gene are very heterogeneous at the DNA levels, occurring in all 18 exons of the gene. However, exon 4 has been found to be the hot spot for mutational events. In this study DNA from 45 Thai subjects with primary hypercholesterolemia was screened for mutations in the hot spot exon 4. The DNA samples were amplified by Polymerase Chain Reaction (PCR) and screened for mutation by Cleavase Fragment Length Polymorphism (CFLP) technique. Identification of mutation was performed by direct sequencing of PCR product. From this screening, one female patient was found to be heterozygous for a novel mutation which was due to a G to T transversion at nucleotide 514. This transversion would change the species-conserved amino acid at codon 151 from charged R group aspatic (GAC) to uncharged R group tyrosine (TAC), termed D151Y. From the same screening strategy, we found that this mutation was absent in 33 healthy normolipidemic subjects. In this index subject, Arg 3500 Gln mutation in apo B-100 gene, causing hypercholesterolemia namely familial defective apo B-100 (FDB), was not found. Therefore, hypercholesterolemia in this index subject was possibly caused by the D151Y mutation in the LDL receptor gene.