Familial lecithin:cholesterol acyltransferase (LCAT) deficiency is a well-defined inborn error of metabolism, where the enzymatic deficiency (LCAT) has been clarified and also the chromosomal defect (chromosome 16q22) is localized. The disease is to-day known all around the world and 50 patients from 26 families are known to-day. Corneal opacities have been found in all patients and appear early in life. The opacities have a characteristic appearance which makes it rather easy to get the correct diagnosis of this disease. Near the limbus, a pronounced opacity of annular shape resembling a marked arcus lipoides senilis occurs. The opacities are composed of numerous minute grayish dots and are localized to the parenchyma and evenly distributed in all layers of the stroma. In polarized light, crystals that may be cholesterol have been seen in both cornea and the fundus. Excess unesterified cholesterol and phospholipid has been found in cornea. The disease is also characterized by slight anemia and proteinuria and sometimes lipemic plasma, but patients without anemia and proteinuria have also been described. All lipoproteins are abnormal in familial LCAT deficiency. The individual lipoprotein fractions are all heterogeneous and characterized by a higher amount of free cholesterol than normal. Rapidly developing renal insufficiency in adult age often appears in this type of familial renal disease. Kidney transplantation may be necessary. LCAT has now been characterized as a glycoprotein with 416 aminoacids + hydrophobic leader sequence of 24 aminoacids and an apparent Mr of 63 kD. Plans exist to proceed with genomic cloning of the LCAT gene from normal DNA and from various patients.