We describe a mutation of the low-density lipoprotein (LDL) receptor gene, designated familial hypercholesterolemia (FH)-Espoo, which deletes exon 15 of the LDL receptor gene. The mutant receptor is predicted to lack 57 amino acids, including 18 serine and threonine residues, which are the sites of the clustered O-linked sugars of the receptor. Studies on 10 carriers of this gene revealed that FH-Espoo is associated with an exceptionally mild form of FH. Thus, in conditions in which cell proliferation was rendered dependent on the function of LDL receptors, lymphocytes from the patients with the FH-Espoo allele had a growth rate intermediate between those from healthy subjects and patients with the FH-Helsinki gene, a mutation known to abolish LDL receptor function. The in vivo fractional catabolic rate of LDL apolipoprotein B was lower than normal in the two FH-Espoo heterozygotes studied. Although higher than those in healthy controls, the serum LDL cholesterol concentrations in patients with the FH-Espoo gene were significantly lower than those in patients with the FH-Helsinki mutation. The thickness of the Achilles tendons was within the normal limits in subjects with the FH-Espoo gene. Our study suggests that moderate varieties of hypercholesterolemia, ie, those not considered to represent FH, may occasionally be due to subtle LDL receptor gene mutations.