Renal lesions of a new case of lecithin-cholesterol acyltransferase deficiency in an 18-year-old male are described. Large mesangial deposits and a sieve-like transformation of the peripheral basement membrane were the main glomerular lesions. Immunofluorescence identified C3 deposits in the mesangium. A heterogeneous pattern of ultrastructural findings was observed by electron microscopy. Thread-like structures with faint cross-striation and irregular tubular structures embedded in an amorphous material were found in mesangial and subepithelial sites. Mesangial areas and peripheral basement membranes showed irregular holes sometimes containing highly osmiophilic lamellar bodies. It is suggested that many mechanisms may be involved in the production of renal lesions induced by the lipoprotein abnormalities characteristic of the disease.