We have characterized the molecular defect causing lecithin:cholesterol acyltransferase (LCAT)-deficiency (LCAT-D) in the LCAT gene in three siblings of Austrian descent. The patients presented with typical symptoms including corneal opacity, hemolytic anemia, and kidney dysfunction. LCAT activities in the plasma of these three patients were undetectable. DNA sequence analysis of polymerase chain reaction (PCR)-amplified DNA of all six LCAT exons revealed a new point mutation in exon IV of the LCAT gene, i.e., a G to A substitution in codon 140 converting Arg to His. This mutation caused the loss of a cutting site for the restriction endonuclease HhaI within exon IV: Upon digestion of a 629-bp exon IV PCR product with HhaI, the patients were found to be homozygous for the mutation. Eight of 11 family members were identified as heterozygotes. Transfection studies of COS-7 cells with plasmids containing a wild-type or a mutant LCAT cDNA revealed that, in contrast to the cell medium containing wild-type enzyme, no enzyme activity was detectable upon expression of the mutant protein. This represents strong evidence for the causative nature of the observed mutation for LCAT deficiency in affected individuals and supports the conclusion that Arg140 is crucial for the structure of an enzymatically active LCAT protein.