Sacroiliitis is the most pathognomonic and earliest manifestation of ankylosing spondylitis. We herein report a 28-year-old female patient who presented with sacroiliitis as an initial manifestation of acute myelogenous leukemia (AML). She had a 3-month history of anemia and walking difficulty. Bone marrow findings revealed an increase of blasts with trilineage dysplasia. Although she was initially diagnosed with myelodysplastic syndrome (MDS), blasts rapidly increased and AML developed 1 month after the diagnosis of MDS with Sacroiliitis. Induction chemotherapy failed to induce a complete remission of AML, but it did effectively treat the sacroiliitis. However, the sacroiliitis relapsed when the leukemia cells progressed thereafter. Oral corticosteroids helped ameliorate the sacroiliitis. She underwent bone marrow transplantation (BMT) from an HLA-identical sister during a nonremission period; however, the leukemic cells began to rapidly increase from day 30 after BMT. The close relationship between the occurrence of sacroiliitis and AML suggested that autoimmune sacroiliitis was a paraneoplastic phenomenon of AML in this patient. Although autoimmune disorders develop in a substantial number of MDS patients, they are rarely observed in de novo AML. No previous report has described sacroiliitis as the initial manifestation of de novo AML.