Patient: a 9-year, 7-month-old Japanese girl (height 127 cm, body weight 33 kg, body mass index 20.5 kg/m2). Primary complaint: severe deformity of the femur. Past medical history: no notable history. The patient was referred to our facility with complaints of progressive deformity of her right femur associated with an SBC and pathological fractures. The girl experienced a pathological fracture of her right femur due to bone tumor when she was 4-years, 6-months old, which her previous physician treated with lesion curettage and fixation. Pathological findings confirmed the presence of an SBC. Bone healing was confirmed 6 months later, at the age of 5 years. The fixator was removed and steroids were injected simultaneously with an artificial bone graft into the lesion. She wore a functional brace after the surgery. However, 1 week after removing the fixator, a new fracture developed in the same location of the bone, following a minor external injury. After a 5-week trial of conservative treatment using steel wire skeletal traction, she underwent fixation with application of a hip spica cast. Five months after the second fracture, at the age of 5 years and 5 months, weight-bearing on the affected limb was progressively initiated, and she was discharged with full weight-bearing status at the age of 5 years and 7 months. She was monitored as an out-patient. Subsequently, she developed a fracture again at the age of 6 years and 4 months, following a fall. She was once again treated with a steel wire skeletal traction. Once bone healing was achieved, the inside of the cyst was curetted and a cannulated screw was inserted to reduce localized pressure. Approximately 6 weeks after surgery, a hip spica cast was applied, which was followed by the use of a functional brace. During the follow-up period, severe bowing of her right femur developed, which was progressive. At the age of 9 years and 6 months, the femur fractured, with the fracture originating at the cannulated screw. The screw was subsequently removed and the patient was referred to our facility at the age of 9 years and 7 months. On first examination at our facility, the femur was deformed, with a bowing of approximately 90° in the central area of the femur and an internal rotation of 60°, with incomplete fractures observable in the same area. We proceeded with resection of a 7-cm portion of the bone, which included the SBC, corrected the alignment and applied an Ilizarov fixator to gradually lengthen the thigh. We also cut a portion of healthy bone from the proximal femur (approximately 10 cm from resected lesion), for use as bone extension at the site of resection. One week after surgery, extension of the bone was initiated at a speed of 1 mm/day, completing the process in 4 months. Fixation was then maintained until the callus matured. Once callus maturation was achieved, the external fixator was gradually removed to prevent re-deformation. On histopathologic examination of the surgically resected bone, fibrillation of the medullary cavity was increased, and a small formation, identified as inorganic-like material, was scattered throughout; these findings were consistent with a fracture due to a bone cyst. At present, 3 years after surgery, correction of the deformity has been maintained, and our patient does not experience any limitations in daily activities or regular exercise.