METHODS: Forty-nine dogs with spontaneously occurring osteosarcoma underwent limb-sparing surgery after preoperative therapy consisting of intra-arterial cisplatin alone or intra-arterial cisplatin in combination with doses of radiation from 20-40 Gy in 10 fractions. All resections were marginal, and the defect was repaired with a cortical allograft. RESULTS: Local tumor control was strongly influenced by the percent necrosis in the excised specimen. Overall, the estimated 1-year local recurrence rate was approximately 32% (by life-table estimate). Dogs with less than 75% necrosis had an estimated 1-year recurrence rate of 65%; those with greater than 75% necrosis had an estimated 1-year recurrence rate of 15% (P = 0.004, by log-rank test). Local recurrence was influenced by the radiation dose. Dogs receiving 28 Gy or less had an estimated 50% 1-year recurrence rate, and those receiving 32 Gy or more had an estimated 8% 1-year recurrence rate (P = 0.03, by log-rank test). Normal host bone more frequently became necrotic at doses of 36 Gy or more. CONCLUSIONS: Intra-arterial cisplatin in combination with moderate doses of radiation (32 Gy) can achieve a high percent tumor necrosis while maintaining host bone viability. Survival was limited by distant metastasis but was lengthened by treatment compared with some earlier findings.