Aim: We studied the frequency of Achilles tendon xanthoma (ATX) in patients with acute coronary syndrome (ACS). Furthermore, we investigated the differences in clinical findings between ACS patients with and without ATX. Methods: Patients with ACS (n=335) were admitted to the coronary care unit of Nippon Medical School between July 2011 and December 2014. Informed consent for the measurement of Achilles tendon thickness (ATT) on a radiograph was obtained from 228 patients without tendon rupture. ATT of each side was measured on the radiograph in patients with ACS and in those with acromegaly (n=18), non-familial hypercholesterolemia (non-FH
n=96), and familial hypercholesterolemia (FH
n=31). Results: ATT of the right and left side in ACS patients were 6.9±1.3 and 7.0±1.6 (mm
mean± SD). In acromegaly, non-FH, and FH patients, ATT of the right/left side were 6.6±1.1/6.7±1.1, 6.2±0.9/6.6±1.0, and 9.4±3.3/10.0±3.1, respectively. ATX (ATT ≥9 mm) was found in 26 (11.4%) patients with ACS. Patients with acromegaly and non-FH had no ATX, whereas all patients with FH had ATX. No differences in age and serum lipid profiles were observed between ACS patients with and without ATX. The levels of body mass index and glycated hemoglobin of ACS patients with ATX were significantly greater than those in ACS patients without ATX (26.8±4.0 vs. 23.9±3.3, p＜0.05, and 6.9±1.4% 6.3±1.3%, p＜0.05, respectively). Conclusion: This is the first report in which the frequency of ACS patients with ATX was 11.4%. The serum lipid profiles of ACS patients with ATX were similar to those without ATX. In the future, ACS patients with ATX will be diagnosed as having FH.