OBJECTIVE Xanthomas are an essential diagnostic criteria of familial hypercholesterolemia. The objective of this study was to determine if xanthomas of the Achilles tendon can be revealed on sonography when the condition is clinically unsuspected in patients with heterozygous familial hypercholesterolemia. METHODS Ninety-four patients (52 females, 42 males; 12-73 years old; mean age, 44 years) with a proven diagnosis of heterozygous familial hypercholesterolemia were studied. Achilles tendons of these patients were clinically evaluated and examined with sonography. Size and echo structure of the Achilles tendons were categorized as normal, having hypoechoic nodules (grade 1), or diffusely hypoechoic with a heterogeneous echo structure (grade 2). RESULTS Sixty-two patients had positive physical examination findings for Achilles tendon xanthomas. Fifty-seven (92%) of these patients had abnormally large tendons (> or =7.1 mm) on sonography. Grade 1 or grade 2 echo structure compatible with xanthomatosis was found in at least one Achilles tendon of all 62 patients. Of the 94 patients in the study group, 32 patients had negative or indeterminate physical examination findings for Achilles tendon xanthomas. Sonography showed that two (6%) of these patients had an enlarged (> or =7.1 mm) Achilles tendon. Grade 1 or grade 2 echo structure compatible with xanthomatosis was found in 26 (81%) of these 32 patients. CONCLUSIONS Sonography is significantly more sensitive than physical examination for the detection of Achilles tendon xanthomas in patients with heterozygous familial hypercholesterolemia and normal-sized Achilles tendons. Our study suggests that sonography may play an important role in the early diagnosis of heterozygous familial hypercholesterolemia.