Abstract Introduction Juvenile spondyloarthritis is a rare condition in which differential diagnosis is the key step that every practitioner must take to avoid missing other pathologies that can be serious and require early and specific management, such as malignancy. In case of doubt, referring the patient to a referent seems to be the best attitude to adopt. Aim We report a case of a girl who was misdiagnosed as juvenile spondyloarthritis when it was acute lymphoblastic leukaemia with vertebral bone involvement. Observation A 7-year-old girl was referred to us for better management of her juvenile spondyloarthritis refractory to usual treatments. The child first consulted us for excruciating back pain. Diagnosed as juvenile spondylitis, the patient was placed on NSAIDs and then corticosteroids but without any response. On the contrary, her general condition got worsen and her back pain increased. At our level, further questioning found out that the girl had asthenia, anorexia, excruciating low back pain radiating to the lower limbs with neck pain, and abdominal pain that had been evolving for 40 days. On examination, the child was febrile at 40 °C with mucocutaneous palor, she had hepatosplenomegaly with painful palpation of the lumbar vertebrae. On biological examination, the blood count formula showed normochromic normocytic anaemia, hyperleukocytosis with a normal platelet count. Blood smear; presence of 07% blasts. Medullogram: infiltration of 66% of young lymphoid cells. Immunophenotyping was in favor of B-II acute lymphoblastic leukaemia (B-II ALL). The MRI (magnetic resonance imaging) of the spinal cord showed a total and diffuse infiltration of all the dorsal-lumbar and pelvic vertebral parts, resulting in a real spinal cord replacement secondary to a probable hemopathy. The patient was transferred to the pediatric oncology department and commenced chemotherapy. Conclusion Through our observation, JIA is a diagnosis of exclusion and must be considered only after a complete investigation of the other pathologies which require a specific management.