Achilles tendon thickness (ATT) of 112 patients with familial hypercholesterolemia (FH) with and without ischemic heart disease (IHD) was measured radiographically and was compared with that of normal subjects. The mean and SD of serum cholesterol in the heterozygotes (107 cases), the homozygotes (5 cases) and the normal subjects (36 cases) were 347 +/- 63, 589 +/- 69 and 187 +/- 30 mg/dl, respectively. The mean and SEM of ATT in the heterozygotes, the homozygotes and the normal subjects were 12.5 +/- 0.4 mm, 18.6 +/- 6.6 mm, and 6.3 +/- 0.2 mm, respectively. Cutaneous xanthomas were observed in 34 out of 112 patients (30.4%). Increased ATT was observed in 95 (84.8%). IHD was diagnosed in 39 (34.8%). The ATT of FH with IHD was significantly thicker than that of FH without IHD (P less than 0.05) and that of normal subjects (p less than 0.001). Thus, the increased ATT evaluated by x-ray was the earliest clinical sign of FH and the measurement of ATT seems to be a useful adjunctive procedure for detecting familial hypercholesterolemic patients and predicting IHD in them.