The pattern of tumor recurrence was assessed in 24 dogs receiving cobalt radiation therapy for nasal carcinoma. Dogs were evaluated using nasal cavity computed tomography prior to treatment, and at 1, 3, 6 and 12 months after treatment, and at 6-month intervals thereafter if still alive. Dogs were treated with various combinations of total dose, and fraction size. Total doses were normalized to equivalent doses given in 3.0 Gy fractions. The extent of tumor regression or duration of tumor control were not dependent on absolute total dose, normalized total dose, or tumor type. The median duration of local control in all dogs was 312 days. Marked tumor regression was observed in 11 of the 24 dogs. Median duration of local control was significantly longer in dogs with marked tumor regression in comparison to dogs without tumor regression; 389 vs. 161 days respectively. When tumor recurrence was documented in dogs having tumor regression, the location of the recurrence was in the nasal cavity. No tumor recurred in a sinus or periorbital region, and only one geographic miss was detected. Tumor recurrence in the irradiated volume, including dogs with and without marked regression, was documented in 13 of the 24 dogs. The high local failure rate, coupled with the recurrence pattern in these dogs, suggests there may be an opportunity for improvement in local control through use of shrinking field techniques.