A 17-year-old male was admitted to the hospital because of fever, low back pain and knee pain which had started a couple of months ago. Pelvic CT-scan confirmed left sacroiliitis. Complete blood count revealed pancytopenia.A bone marrow biopsy and aspiration was performed under local anaesthesia. The pathologist reported Acute Lymphoblastic Leukaemia. The reported patient is the first case of acute lymphoblastic leukaemia, accompanied by sacroiliitis.