A 12-year-old girl was admitted to the paediatric department due to a short history of severe pain in the right hip, inability to walk and fever, 39 °C. She had had no previous trauma or ongoing infection elsewhere. Examination showed a septic girl with pain of the right sacroiliac joint. Laboratory findings showed a marginally raised white blood cell count and C-reactive protein. A magnetic resonance imaging was performed and showed effusion of the right sacroiliac joint without root compression. Three days later blood cultures showed group B streptococci and the patient received intravenous antibiotic treatment for two weeks and continued with oral antibiotics for four weeks.