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A retrospective study was undertaken in which cytological features of basal cell tumours from 18 dogs and 12 cats were examined. Fine-needle aspiration biopsy was performed for cytological examination and diagnosis was confirmed on histopathological examination of excised tumour specimens. Cytological smears were analysed for occurrence of cystic areas, as well as frequencies of squamous cells, fibrocytes, fibroblasts, neutrophils, lymphocytes, mast cells, melanocytes and basal cells. Basal cell aggregates were examined for indicators of malignancy, evidence of basaloid cell structure, well defined cellular borders and cell aggregation patterns. Cellularity and quality of smears was moderate to good, although, in six aspirates, cellularity was poor due to a high amount of degenerate material within the tumours. Fifteen of the smears revealed between one and three nuclear criteria of malignancy. However, despite the less well differentiated appearance of basal cells that was sometimes seen, the tumours were considered benign based on histopathological examination. Basal cell tumours are therefore likely to be underdiagnosed on cytological examination due to non-specific features. The occurrence of other cell populations and lack of cells with basaloid structures and linear aggregation patterns might further confuse the tumour diagnosis. The authors conclude that the cytological diagnosis of basal cell tumours can be based on the criteria described in the present study.