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Background and purpose The natural pattern of bone healing in large bony defects following curettage alone as treatment of benign bone tumors around the knee is not well reported. We analyzed the outcome in 146 patients. Patients and methods 146 patients with over 18 months of follow-up who underwent curettage without bone substitute filling or bone grafting for a benign tumor in the distal femur or upper tibia were included. The mean diameter of the defects following curettage was 5.7 (1.3–11) cm and the estimated average volume was 63 (1–240) cm3. The plain radiographs before and following curettage were reviewed to establish the size of the initial defect and the rate of reconstitution and infilling of the bone. The time to full weight bearing and any complications were recorded. Results There was a variable rate of infilling; some defects completely reconstituted to a normal appearance while some never filled in. In 88% of the cases, no further intervention after curettage was required and the mean time to full weight bearing was 6 weeks. The risk of subsequent fracture or the late development of osteoarthritis was strongly related to the size of the cyst at diagnosis, with cysts of > 60 cm3 (about 5 cm in diameter) having a much higher incidence of complications. Interpretation This study demonstrates the natural healing ability of bone without any adjuvant filling. It could be used as a baseline for future studies using any sort of filling with autograft, allograft, or bone substitute.