A large infiltrating mast cell sarcoma in a dog, which had been refractory before surgical excision, was controlled 2 months after completion of a combined radiotherapy and hyperthermia regimen. Treatment resulted in rapid tumor necrosis and resultant ulceration of adjacent skin. Ulceration was transient, resolving concurrently with tumor control. Radiation was administered as 3.5-Gy fractions 3 times/week, resulting in a total dose of 45.5 Gy in 13 treatments. Hyperthermia (44 C for 30 minutes) was given 4 to 5 hours after radiotherapy, once a week during the first 3 weeks of treatment.