We studied the effect of th bite acid sequestrant colestipol, alone and in combination with clofibrate or niacin, in patients with heterozygous familial hypercholesterolemia who were given a diet low in cholesterol and saturated fat. With colestipol alone, mean cholesterol levels in serum decreased 16 to 25 per cent. The addition of clofibrate produced a total mean decrement of only 28 per cent. In contrast, serum cholesterol levels fell 45 per cent when colestipol as combined with niacin. Low-density-lipoprotein (LDL) cholesterol decreased 55 per cent with colestipol and niacin, whereas high-density-lipoprotein (HDL) cholesterol increased. Mean LDL cholesterol was lower in patients given this regimen than in matched normal controls eating an unrestricted diet. Tendinous xanthomas, measured by quantitative xeroradiography, were significantly reduced (P < 0.01), indicating that this regimen mobilized cholesterol from tissue pools with slow turnover. Colestipol plus niacin promises to be useful in the treatment of patients at high risk from elevated levels of LDL.