Abstract Familial hypercholesterolemia (FH), caused by many different mutations in the low-density lipoprotein (LDL)-receptor gene, invariably leads to severe premature coronary heart disease (CHD) in homozygous individuals. Heterozygous FH patients are less severely affected but are still at increased risk of CHD in most populations. Although FH homozygotes in China are affected similarly to those elsewhere, heterozygotes are not detected in the general population and obligate heterozygotes are often not hypercholesterolemic by Western standards. Mutations in the LDLreceptor genes of 10 homozygous FH patients from the Jiang-su province of China and their heterozygous parents were analyzed. These include one large and two minor deletions and eight point mutations: four are predicted to introduce a premature stop codon, five to result in a single amino acid substitution or deletion, and one to produce a protein with an abnormal cytoplasmic tail. Expression of the mutant LDL-receptor cDNAs in vitro confirmed that these mutations impaired LDL-receptor function and that several would cause a receptor-negative phenotype. Thus, the lack of clinical expression in obligate FH heterozygotes is not due to unusually "mild" mutations in the LDL-receptor gene, and other genetic or environmental factors must therefore be important in determining phenotypic expression.