Abbreviations PS Pyogenic sacroiliitis SI SacroiliacalPyogenic sacroiliitis (PS) is an uncommon joint infection. Clinical presentation may be misleading, since it can resemble more common conditions such as sciatica, acute abdomen or septic arthritis of the hip.We report a 14-year-old girl with PS. She presented at another hospital with fatigue and a six-day history of progressive low back pain radiating down to the right knee. Plain X-ray examination and computed tomography (CT) scans of the spine were normal. She was discharged with analgesic therapy. As she developed high fever over 40°C, worsening pain and immobility, she was admitted to our emergency department. Although the temperature at that moment was only 37.2°C, she appeared ill. The right sacroiliac joint and buttock were tender. Internal and external rotation, extension and flexion of the hip were decreased and painful. Compression of the sacroiliac joint was very painful. Further clinical examination was normal.Blood examination revealed an erythrocyte sedimentation rate of 54 mm/hour and a C-reactive protein (CRP) of 16.5 mg/dl (normal<0.5 mg/dl). Blood cultures were negative.Bone scintigraphy revealed increased uptake of 99m Tc-MDP in the right sacroiliac joint, suggesting sacroiliitis. This was confirmed by magnetic resonance imaging (MRI). There was oedema of the ileum and infiltration of the surrounding soft tissues anterior of the sacroiliac joint, with an inflammatory fluid collection extending to the pre-sacral region. Intravenous flucloxacillin treatment was started. The fever disappeared within 2 days and the pain diminished. By day 7, the CRP had fallen to 0.5 mg/dl. Follow-up MRI examination on day 9 showed beginning resolution of the collection and the infiltration of the surrounding soft tissues. She was then switched from intravenous to oral antibiotherapy and discharged. Oral flucloxacillin was continued for 6 weeks.Pyogenic sacroiliitis accounts for 1.5% of all joint infections [6]. The diagnosis is often initially overlooked because of poorly localising initial symptoms, the relative rarity of the disorder, the physician's low suspicion and inaccurate physical examination [5]. The pathophysiology of PS is presumed to be the haematogenous spread of bacteria but, in the majority of cases, no primary source is identified [6]. Predisposing risk factors in adults are intravenous drug abuse and infections of the skin, respira-