Thirty-five canine cutaneous mast cell tumours (CCMCTs) were analysed by computerized nuclear morphometry. In each case, the nuclei of at least 100 neoplastic cells were measured, and the mean nuclear area (MNA), mean nuclear perimeter (MNP) and mean nuclear form factor (FF) were calculated. Significant differences in respect of MNA and MNP occurred between tumours of grades I and III and between those of grades II and III (P<0.01) but not between tumours of grades I and II (P>0.01). No significant differences in respect of FF were observed between tumours of different grades. The results obtained indicate that nuclear morphometric analysis may assist in the grading of CMCTs.