The diagnosis of familial hypercholesterolemia (FH) is frequently made on clinical grounds and detection of tendon xanthomas is crucial for that. In order to clarify whether ultrasonography (US) can be used as a reliable and practical method for detection of Achilles tendon xanthomas in FH, the Achilles tendon thickness in the sagittal section was examined by US in 15 patients with heterozygous FH and 34 normocholesterolemic subjects. US visualized clearly the anterior and posterior borders of the Achilles tendon. The Achilles tendon thickness determined by US correlated with that measured by conventional radiography (r = 0.99). The mean values +/- SD of the Achilles tendon thickness determined by US were 4.5 +/- 0.5 mm in the normal controls and 11.9 +/- 5.1 mm in the patients and the difference was significant (P less than 0.001). In 13 of 15 patients, US visualized thickened Achilles tendons with convex shape in the sagittal section. All the thickened Achilles tendons revealed by US were confirmed by radiography. The data indicate that US can detect Achilles tendons thickened by xanthomas. We conclude that US is a useful aid in the clinical diagnosis of FH.