Spontaneous necrosis in osteosarcoma. The percentage of necrosis in a primary osteosarcoma after the patient has received preoperative chemotherapy is prognostic and is usually used to select subsequent chemotherapy. However, the percentage of necrosis that occurs spontaneously, without preoperative chemotherapy, has not been adequately studied. The examination of histologic macrosections of 76 osteosarcomas from patients who had not received preoperative treatment and of 20 patients who had received preoperative chemotherapy revealed a significant difference in the percentage of necrosis. There was minimal spontaneous necrosis, but necrosis after preoperative chemotherapy was usually extensive. The larger the tumor, the greater the percentage of spontaneous necrosis; however, size did not correlate with the percentage of necrosis with preoperative chemotherapy. Survival among those patients who did not receive preoperative treatment correlated with the size and percentage of necrosis. Necrosis in an osteosarcoma in a patient who has received preoperative chemotherapy can be considered the result of chemotherapy and not a spontaneous event. 