To define the metabolic defect in Tangier disease, we studied the kinetics of [¹²⁵I]-high-density lipoprotein apolipoproteins (apolipoproteins A-I and A-II) in 11 normal subjects, two obligate heterozygotes, and two homozygotes. Mean synthesis of apolipoproteins A-I and A-II was 8.24 mg per kilogram per day in the normal group, 7.94 in heterozygotes and 3.66 in homozygotes. The mean plasma-residence time for both apolipoproteins was 5.21 days in the normal subjects, 3.41 days in heterozygotes, and 0.52 days in homozygotes. In normal subjects and heterozygotes the apolipoproteins were catabolized at similar rates, whereas in homozygotes apolipoprotein A-I was catabolized at a much greater fractional rate than apolipoprotein A-II. These findings indicate that the deficiency of these apolipoproteins in Tangier disease is largely due to rapid and altered catabolism. (N Engl J Med 299:905–910, 1978)