The Achilles tendons of 43 men and women were examined by conventional radiography and by computed tomography (CT). Eleven subjects were normolipidaemic and 32 had familial hypercholesterolaemia, of whom 19 were considered to have polygenic hypercholesterolaemia and 13 monogenic. Achilles tendon xanthomata were identifiable by both radiography and CT. In assessing the size of xanthomata, CT had advantages since the junction between muscle and tendon was easily identified and all the dimensions and the cross-sectional area of the tendon could be measured. The area of Achilles tendons was closely correlated with body height in normolipidaemic men and women and patients with polygenic hypercholesterolaemia. In monogenic hypercholesterolaemia this relationship was lost. The tendons of the normolipidaemic group and of patients with polygenic hypercholesterolaemia were indistinguishable, whereas those of patients with monogenic hypercholesterolaemia were significantly larger. Neither the maximum nor the average CT attenuation values of the tendons differed significantly between the 3 groups. Of the patients with monogenic hypercholesterolaemia, those whose Achilles tendons were less than 200 mm2 in area had received treatment longer than those with larger tendons. CT shows promise as a means of assessing the therapeutic response of patients with monogenic familial hypercholesterolaemia.