Ninety cats were irradiated for treatment of squamous cell carcinoma of the nasal plane. The 1- and 5-year progression-free survival rates were 60.1 +/- 5.5% and 10.3 +/- 6.2%, respectively. Analysis of progression-free survival times revealed that clinical stage and tumor proliferative fraction (estimated by the use of a proliferating cell nuclear-antigen immunohistochemical method) had significant prognostic value. Conversely, coat color, presence of multiple facial carcinomas, histologic grade, and feline immunodeficiency virus infection status were not found to have prognostic value. Acute radiation reactions were mild and self-limiting. Severe chronic radiation reactions were more frequent in cats infected with feline immunodeficiency virus. Results of the study indicated that cats with squamous cell carcinoma of the nasal plane benefit from radiotherapy and that treatment might be improved by increasing the radiation dose as well as altering the dose-fractionation scheme.