We observed four patients on whom bone allograft sterilized with ethylene oxide gas was used intraoperatively to fill bone defects after curettage. Average age was 16.7 years. They consisted of two males and two females. Two had single bone cyst, one giant cell tumor, and one chondroblastoma. All cases showed swelling and redness around their wound, and discharge from an early period after surgery. Although postoperative infection was suspected, all cultivation was found to be bacteria free. There was no elevation in C-reactive protein from the second week after surgery. Three cases underwent surgical treatment. A case receiving conservative treatment with immunosuppressant agent showed good results of gradual wound healing. We suggest inflammation was induced by residual ethylene oxide of the freeze-dried bone.