Acute lymphoblastic leukaemia (ALL) is the most common malignancy in children, representing one third of all paediatric malignancies. Patients are often at high risk for complications due aggressive chemotherapy regimes required for treatment. Musculoskeletal complications include septic arthritis, osteonecrosis, osteoporosis, avascular necrosis and bony ankylosis. We report the case of a 16-year- old boy with ALL who developed osteonecrosis of multiple bones on a background of septicaemia, resulting in bony ankylosis of both hips and knees. He was treated with bilateral conversion of kylosed hips (one hip to total hip replacement, the second hip to Girdlestone arthroplasty) and bilateral ankylosed knees to total knee replacements. He remained well in remission five years after the last surgery. Our case highlights he possible musculoskeletal complications of ALL.