A 9-year-old boy with thigh pain, high fever, hyperleukocytosis and positive blood cultures forStaphylococcus aureus had a left obturator internus phlegmon. Lack of evidence for hip infection or osteomyelitis in a child with thigh pain, high fever and hyperleukocytosis points to a possible infection of pelvic structures. Abscess or phlegmon of the obturator internus muscle is a very rare condition. The most frequent agent isStaphylococcus aureus. CT of the pelvis is the procedure of choice to show diffuse swelling of the muscle or collection of pus. Conservative treatment with adequate antibiotics is the treatment of choice.