A 1 1/2-year-old castrated Boxer with a progressive left forelimb lameness of 1 month's duration had a solitary immunoblastic B-cell lymphoma of the proximal portion of the ulna. This primary lymphoma of bone was successively treated with orthovoltage radiation (45 Gy divided into 10 fractions over 22 days). The dog gradually regained normal gait function and did not develop any local recurrence over 34 months. In dogs, primary lymphoma of bone is a rare primary bone tumor, and the ulna is an unusual location for extranodal lymphoma. Local administration of radiation appears to be an effective treatment, allowing retention of normal function.