To elucidate the deranged metabolism of Tangier disease, the turnover of esterified cholesterol, and the activities of lipoprotein lipase and of lecithin-cholesterol acyltransferase (LCAT) were studied in a patient with typical findings. The plasma concentration of high-density lipoproteins, assessed by electrophoretic, immunologic and ultracentrifugal means, was greatly reduced. Of a total plasma cholesterol of 59 mg per 100 ml (70 per cent esterified), only 7 mg was present in high-density lipoprotein. In vivo turnover of plasma esterified cholesterol, measured after injection of Radio-labeled mevalonic acid, was 48 mg per hour, similar to values obtained in normal subjects. Postheparin plasma lipoprotein lipase activity was 0.071 μmoles of free fatty acids released per minute per milliliter of plasma, compared to a normal value of 0.218. LCAT activity was 2.5 μg of cholesterol esterified per milliliter per hour of incubation, about half the normal value.