The average number of nucleolar organiser regions per cell has previously been shown to correlate well with histological grading techniques for a variety of neoplasms in man, and may thus be of value as an aid to post-surgical prognosis. In this study 50 spontaneously arising, subcutaneous canine mast cell tumours were graded and the histological grade compared with the mean AgNOR count. For well differentiated neoplasms the mean count was 1.4 per cell compared with 6.3 for poorly differentiated neoplasms, while tumours of intermediate differentiation had a mean count of 3.2 per cell. Subsequent follow up studies revealed that the AgNOR count was an accurate prognostic indicator, 73% of dogs with a high mean count (greater than 4.9) being destroyed from tumour related disease compared with 33% with an intermediate count (1.7-4.8). No dog with a count of less than 1.7 has been destroyed because of tumour recurrence to date and the AgNOR count has proved to be a better and more objective prognostic indicator than either histological tumour grade or mitotic index. Since most dogs which develop recurrent mast cell tumours do so within 6 months of initial surgery, an assessment of the predictive value of AgNORs can be obtained more quickly in canine tumours than for comparable human neoplasms.