Patients with leukemia can be presented with monoarthritis without any hematologic abnormalities. These patients may be misdiagnosed with juvenile idiopathic arthritis (JIA), and the main treatment can also be delayed. An 11-year-old girl was admitted to our pediatric rheumatology outpatient clinic with a 4-week history of swelling in the left ankle. JIA was considered as a preliminary diagnosis after the antinuclear antibody was found to be positive, and non-steroidal anti-inflammatory drug was started. Diffuse bony edema was observed in the talus, navicular, cuboid, and cuneiform bones in magnetic resonance imaging of the left ankle. Despite the treatments, the patient's joint pain increased. There were no abnormalities in repeated peripheral blood smears. On week 3 of follow-up, after bicytopenia was revealed in complete blood count, bone marrow biopsy was performed, and she was diagnosed with precursor B cell acute lymphoblastic leukemia. We presented this case to emphasize that malignancies must be evaluated in the differential diagnosis of patients with arthritis.