Studies were made on the abnormality of lipoprotein metabolism and its cause in 10 patients with Werner's syndrome. Seven of the 10 patients had hypercholesterolaemia (above 250 mg dl-1). Six of the seven patients with hypercholesterolaemia had thickened Achilles' tendons (greater than 9 mm). A significant positive correlation (P less than 0.01) was found between the serum total cholesterol levels and the thickness of Achilles' tendons in these 10 patients, suggesting that the substance precipitated in the thickened tendons is derived from serum cholesterol. Some first-degree relatives of three patients with both hypercholesterolaemia and xanthoma-like thickening of Achilles' tendons also suffered from hypercholesterolaemia. Moreover, the low density lipoprotein (LDL) receptor activities in peripheral lymphocytes of five patients with both hypercholesterolaemia and xanthoma-like tendons were significantly (P less than 0.001) lower than those of controls, whereas the LDL receptor activities of two patients without hypercholesterolaemia were almost the same as those of controls. These findings suggest that, at least these five patients with lower lymphocyte LDL receptor activities, and probably another patient with both hypercholesterolaemia and xanthoma-like thickening of Achilles' tendons suffered from familial hypercholesterolaemia (FH). If this is the case, this high frequency of association of Werner's syndrome with FH (in six of 10 patients) suggests some relationship between these two diseases.