Seventy-seven dogs with malignant tumors of the nasal and paranasal cavities were treated by use of radiotherapy. The tumors included carcinomas (58) and sarcomas (19). Radiographic findings, including site of involvement and tumor extension, were the basis of clinical staging. Staging was performed according to the tumor, node, metastasis staging of the World Health Organization, and a modified staging scheme based on prognostic factors that seemed to correlate best with response to treatment. All irradiations were done with a telecobalt 60 unit. Fifty-six dogs were treated with irradiation alone, and 21 had partial tumor resection prior to radiotherapy. Treatment dose was 48 Gy (minimal tumor dose) administered on a Monday-Wednesday-Friday basis at 4 Gy/fraction over 4 weeks. The irradiation technique emphasized rostral field with a generous treatment volume. Duration of follow-up after irradiation ranged from 1 month to 61 months. The 1- and 2-year overall survival rates were 60.3% and 25%, respectively, and the 1- and 2-year relapse-free survival rates were 38.2% and 17.6%, respectively. Results of histologic examination and our modified staging scheme were significant (P = 0.02 and P = 0.04, respectively) prognostic factors of relapse-free survival. Conversely, tumor site, tumor extension, World Health Organization clinical stage, and cytoreductive surgery prior to irradiation did not affect the outcome of treatment. According to our modified staging scheme, dogs with stage-2- disease have a poorer prognosis than dogs with stage-1 disease, with a relative risk of relapse 2.3-fold higher. Dogs with carcinoma had a poorer prognosis than dogs with sarcoma (predominantly chondrosarcoma) with a relative risk of relapse 3.3-fold higher.(ABSTRACT TRUNCATED AT 250 WORDS)