Patients with heterozygous familial hypercholesterolaemia (FH) are at high risk for the development of coronary artery disease. Achilles tendon xanthomata are often the first clinical manifestation of FH, but are seldom palpable earlier than during the third decade. Twenty-one FH children aged 3-18 years underwent high-frequency ultrasound examination of the achilles tendon. Hypoechoic infiltration of the normal tendon structure was demonstrated in 8 of 21 (38%) of the FH children. The findings were similar in boys and girls. Control subjects (n = 68) aged 1-25 years had no sonographically detectable tendon abnormalities. The thickness of the achilles tendon of the FH children was (mean +/- SD) 7.1 +/- 1.5 mm (range 5-10 mm). The respective values for the controls were 5.8 +/- 1.0 mm (3-7 mm. We conclude that ultrasound examination sensitively detects cholesterol accumulation in the achilles tendon of FH children before tendon xanthomata are clinically evident.