AIMS: Achilles tendon (AT) xanthomas are a specific physical finding of familial hypercholesterolemia (FH) and AT thickness has been used for its diagnosis and evaluation of its severity. Recently, we reported that the AT of FH patients was softer than that of non-FH patients and the combined use of a cut-off value for AT softness with that for AT thickness improved diagnostic accuracy. However, an association between AT softness and severity of atherosclerosis has not been reported. Accordingly, the present study aimed to investigate whether AT softness was associated with carotid atherosclerosis and presence of atherosclerotic cardiovascular disease (ASCVD) in FH. METHODS: The AT of 176 genetically diagnosed FH patients and 98 non-FH patients was examined to measure AT thickness and the elasticity index (EI) as an indicator for assessing AT softness using ultrasonography. RESULTS: Increased age was associated with AT softness, and overweight was negatively related to AT softness. There were significant inverse correlations between EI and maximum and mean intima-media thickness (IMT) within the common carotid artery only among FH patients. In multiple linear regression analysis, although the relationship between EI and mean IMT was attenuated, the association between EI and maximum IMT remained robust. In logistic regression analysis adjusted for age, sex and traditional cardiovascular risk factors (smoking history, presence of hypertension, presence of diabetes mellitus, overweight, LDL-cholesterol, HDL-cholesterol, and Log triglycerides), EI was associated with presence of ASCVD (Odds ratio per 1-SD increase, 0.37;95% CI, 0.15 - 0.86; P=0.0252). CONCLUSION: The degree of lipid deposition in the AT of FH patients could be assessed by its thickness as well as its softness. AT softness is not only useful in diagnosing FH but is also associated with the severity of carotid atherosclerosis and presence of ASCVD. In addition, these findings suggest that AT softness would be helpful in risk assessment for FH patients.