Objective: The cutoff value on radiography for judging Achilles tendon xanthoma (ATX), one of the diagnostic criteria for familial hypercholesterolemia, was altered from 9 mm to 8/7.5 mm (men/women) in the new guidelines from the Japan Atherosclerosis Society. The present study investigated whether or not ATX newly identified by the revised criteria, i.e. Achilles tendon thickness (ATT) of ≥ 8 mm and < 9 mm in men, is associated with coronary lesion severity in premature coronary artery disease (CAD) patients.<br>Methods: Eighty-seven male subjects with a history of acute coronary syndrome (ACS) under 60 years old and with a maximum ATT for the right and left sides of < 9 mm were enrolled. The severity of the coronary lesions was assessed using the SYNTAX score derived from coronary angiography at the onset of ACS.<br/>Results: Nine patients (10%) had a maximum ATT ≥ 8 mm (ATX group) among the present subjects. The SYNTAX score was significantly larger in the ATX group than in the non-ATX group (18.3 ± 9.8 vs. 12.3 ± 7.3, P = 0.028). The ATX group also had an increased body mass index (BMI) and C-reactive protein (CRP) level. In all subjects, the maximum ATT was positively correlated with the SYNTAX score (R = 0.337, P = 0.001). A multiple regression analysis revealed that the presence of ATX was significantly associated with the SYNTAX score, independent of confounding factors, including the BMI and CRP.<br>Conclusions: This study demonstrated that ATX, even when mild, was associated with coronary lesion severity in premature CAD. ATX, based on new criteria, may be an independent predictor of CAD progression.<br/>