In this study we evaluated the behaviour of proliferating cell nuclear antigen labelling index (PCNA-LI), mitotic index (MI) and histological features in ten basal cell carcinomas (BCC) and ten squamous cell carcinomas (SCC) of canine skin. PCNA-LI ranged from 2.1% to 4.8% in SCC and from 0.1% to 2.0% in BCC samples. A significant statistical correlation, between PCNA-LI and MI could be demonstrated only in SCC, but not in BCC samples. Furthermore, PCNA-LI was also significantly and consistently greater in the three BCC of solid type and in poorly differentiated SCC. PCNA-LI may provide additional criterion in evaluating the intrinsic malignancy and growth potential of the above tumours.