The purpose of this study was to investigate the ultrasonographic features of flexor hallucis longus (FHL) tenosynovitis in sports players. High-resoluted ultrasound was used to measure the thickness and echogenicity of the flexor hallucis longus at the posteromedial ankle, especially the fibro-osseous tunnel, for 60 sports players (120 feet). They were compared with their asymptomatic feet. There were two group; group 1 was symptomatic feet, 2 was asymptomatic. The 36 males (72 feet) and 24 females (48 feet) had an average age of 21.3 years (range, 18.23 years). Thickness, echogenicity of the tendon on each group were evaluated by the ultrasonography and the results were statistically analyzed. The mean thickness of the FHL on group 1 was 3.4±1.2 mm, 2.1±1.3 mm on group 2. There was significant difference between two groups (p=0.002). The thickness of FHL based on gender and location was not significant different (p＞0.05). For group 1, increased thickness of the FHL and reduced echogenicity, peritendious fluid collection were observed in ultrasonographic findings.Increased thickness (＞3 mm) and hypoechoic lesion, peritendinous fluid collection of the tendon were sonographic findings at the posteromedial ankle in the FHL tenosynovitis. The authors suggested that Ultrasonography is a valuable and alternative tool for the evaluation of FHL tenosynovitis.