We report a 13-year-old boy with a 3-month history of low back pain following a mild trauma. Extensive osteoporosis and vertebral collapses were seen on conventional X-ray and computed tomography scan. Laboratory findings were non-specific. Bone marrow infiltration was observed on magnetic resonance imaging, suggesting a myeloproliferative disorder. Although not diagnostic, marrow infiltration in a child with osteoporosis should raise the suspicion of leukaemia.