This study examines flexor digitorum tendons and A1 pulley in patients presenting trigger fingers using high-resolution ultrasonography (US), determines the accurate causes of trigger fingers, and analyzes the relationship between clinical data and US findings. As much as 50 trigger fingers of 41 patients were examined by high-resolution US, and the US findings were analyzed as tendon thickness, fibrillar echotexture, tendon margin, fluid collection, A1 pulley thickening, tendon sheath cyst, and metacarpophalageal (MCP) joint abnormalities. The affected fingers were compared with the asymptomatic opposite sides. Right thumb and ring finger were the most common fingers involved. Mean thickness of flexor digitorum tendons of the affected fingers was increased significantly, compared with that of the opposite fingers. There were various US findings in flexor digitorum tendons as follows; loss of normal fibrillar echogenic pattern (14%), irregularity or blurring of the tendon margin (62%), and fluid collection in the tendon sheath (16%). Thickening of A1 pulley was noted in 44% of fingers, and mean thickness of A1 pulley was 0.7 mm. As much as 6% of fingers had the sheath cysts and 4% of fingers had abnormalities of MCP joints. The patients with extension difficulty of the fingers had thicker flexor digitorum tendon than those without. The patients with locking fingers had more blurred margin of the tendon than those without. US can detect various lesions in clinical trigger fingers, and some US findings correlated with clinical findings.