A 71-year-old man complained of increased swelling of his left wrist and exercise restriction. The patient was admitted to our orthopaedic outpatient department because of increased swelling and restricted movements of his left wrist for half a month. The patient had no recent history of trauma, except for an injury to the back of the left hand more than ten years earlier that resulted in the flexion of the left hand's fingers in a semi-clenched fist shape. He had had a history of eczema for three years, had been treated with traditional Chinese medicine, and denied a history of tuberculosis. The patient had no special personal and family history. Physical examination revealed a cystic mass on the palmar side of the left wrist with unclear borders and mild tenderness. The left hand's fingers were not weak or numb, and Tinel’s sign was negative. The range of motion of left wrist flexion was 0°-45°. Laboratory tests were normal. The erythrocyte sedimentation rate was 18 mm/L, and the C-reactive protein was 0 mg/L. Ultrasound examination in other hospitals showed a cystic hypoechoic mass on the palmar side of the left wrist, with clear borders, an uneven internal echo, noticeable enhancement of the posterior sound, and spot-like blood flow signals around it. We then performed a magnetic resonance imaging (MRI) examination and found a large cystic mass in the volar flexor tendon and carpal tunnel of the left wrist. The mass was filled with rice-sized particles that showed low signals both on the T1 and T2 weighted images. The left carpal tunnel volume had increased, the median nerve structure was unclear, and the left transverse carpal ligament showed an arcuate bulge. Soft tissue swelling of the distal left forearm, around the wrist and the left palm, was observed, with a patch-like long T1 and high T2 weighted-signal shadow. On histopathological examination of the resected cyst wall, chronic, nonspecific inflammation was observed. The postoperative rheumatoid factor test was normal, at 1.40 IU/mL. The final diagnosis was idiopathic tenosynovitis with multiple rice bodies. Two weeks after the operation, the wound healed, and the stitches were removed. During the twelve-month follow-up period, the symptoms resolved without recurrence.