To clarify the safety margin in excision of bone and soft tissue sarcomas, a new evaluation method for surgical margins was drafted by the Bone and Soft Tissue Committee of the Japanese Orthopaedic Association in 1989. This new evaluation system was applied to 1329 patients with bone and soft tissue sarcomas, of whom 492 were excluded because of insufficient details, leaving 837 patients (901 surgeries) for the current study. Based on the results derived from analyzing these registered surgical margins, predictable safety margins under different conditions could be determined. When preoperative treatment is not done or is ineffective in high-grade sarcoma, a margin greater than 3 cm wide is necessary. When the preoperative modality is effective, a 2-cm wide margin is permissible. However, for recurrent sarcoma, whether low-grade or high-grade, a curative procedure is necessary. For low-grade sarcoma, obtaining an adequate wide margin is essential but partial marginal margins are acceptable at sites where barriers exist. When the margin is insufficient, radiotherapy should be used regardless of the tumor grade. Making a safety margin definitive through this system, an optimum treatment can be obtained and unnecessary adjunctive modalities can be avoided.