Introduction: Many children complain once in their childhood of back pain. Unlike adults, back pain in children is often associated with serious disease. We report on a 7 year old child who presented back pain, weakness and hypercalcemia. Case report: Four weeks before diagnosis, the child experienced two traumas, one falling down the stairs without pain and one week later during a trampoline session with acute and persistant pain. In spite of appropriate analgesia with paracetamol and ibuprofen, back pain worsened with appearance of weakness, night pain and finally impossibility to move. At this moment, the child was addressed for further investigation. On physical examination, the child looked tired, pale, was slightly dehydrated and suffered intense back pain. No neurological deficit was found. Standard X-ray and CT of the spine showed compressive fractures of D4-D6-D7-D8-D11-L1-L4. Laboratory analyses revealed hypercalcemia, hyperuricemia, elevated LDH, and discrete thrombocytopenia. At this stage, a differential diagnosis between histiocytosis and leukaemia was considered. For diagnostic reasons, a bone marrow aspiration and biopsy were performed revealing surprisingly medullary infiltration by small blue round cells (95%), negative for histiocytic or leukemic markers, but positive for rhabdomyosarcoma. The alveolar type was confirmed by cytogenetics (fusion complex PAX3-FKHR with translocation T2;13 ). A small nodule on the postero-lateral face of the left arm proved to be the primary tumor by MRI and biopsy. Hypecalcemia was secondary to the diffuse bone metastasis and needed treatment with hyperhydratation, furosemide and pamidronate for several days. Conclusions: Back pain in children is rarely common, especially in young ones. Persistence of pain, progression, occurrence at night and resistance to analgesia need active search for an underlying disease.