We report a case of primary iliopsoas abscess successfully treated by ultrasonographically guided percutaneous drainage. A 56-year-old man presented at our hospital with lumbago, right-sided back pain, fever (temperature 38.5 degrees C) and chills. On physical examination, we found dark red skin, swelling, and tenderness localized at the right side at the back of his waist. Laboratory examination showed leukocytosis (white blood cell count 9700/mm3) with a leftward shift and elevated C-reactive protein (5.2 mg/dl). Ultrasonography (US), computed tomography (CT), and magnetic resonance imaging revealed a hypodense lesion in the right iliopsoas muscle extending to the subcutaneous tissue. About 50 ml of thick yellow pus was obtained by ultrasonographically guided aspiration drainage. A drain catheter was inserted in the abscess cavity. Laboratory findings improved and clinical symptoms abated rapidly after drainage. On the twenty-first day after drainage, US and CT showed that the abscess was no longer present. The patient was discharged after 32 days of hospitalization. As possible primary diseases causing iliopsoas abscess, such as digestive tract disease, tuberculosis, and osteomyelitis, were not found, we diagnosed the disease as primary iliopsoas abscess. Although surgical drainage has been performed in most reported cases of iliopsoas abscess, this case report shows that ultrasonographically guided percutaneous drainage is also effective for treating primary iliopsoas abscess if it is diagnosed early enough.