Lipid metabolism was investigated in a 4-year-old boy with alpha-1 antitrypsin deficiency (ZZ phenotype) and liver disease. Plasma cholesterol and triglyceride levels were 604 mg/dl and 336 mg/dl respectively. Both parents had normal plasma lipids. Lipoprotein X was present at a concentration of 855 mg/dl and levels of apoproteins A-I, A-II, B and C-III were elevated. The plasma free fatty acid pattern was normal. Plasma cholesterol esterification was greatly depressed. Cholesterol absorption on two occasions was reduced about 13% compared with adult controls. Neutral and total steroid excretion was normal with increased excretion of bile acids. A low-cholesterol, low-fat diet reduced plasma cholesterol to 374 mg/dl and triglyceride to 236 mg/dl in two months. Cholesterol and lipoprotein X concentrations were elevated far out of proportion to the severity of the liver disease (total bilirubin 3.7 mg/dl, SGOT 280 IU/L). This suggests that lipoprotein metabolism in patients with this disorder is unusual and may differ from the derangements seen in other forms of liver disease.