Familial hypercholesterolemia (FH) is a genetic disorder caused by mutations in the low density lipoprotein (LDL)-receptor gene. We found a new mutation in the splice acceptor site of intron 1 of the LDL receptor gene, which is designated as 68-1 G->C according to the nomenclature suggested by Beaudet and Tsui (1993), in a Japanese FH homozygote. She was born from consanguineous marriage and has this mutation as a true homozygous form. Her cultured fibroblasts showed no LDL receptor protein synthesis. This mutation caused activation of a cryptic splice acceptor side in the downstream exon 2, leading to frameshift and appearance of premature in-frame stop codon. The mutation was detected by Dde I restriction enzyme. The identical mutation was not found among 24 patients with homozygous and 120 patients with heterozygous FH. The mutation was very rare among the Japanese population.