Detectable p53 immunostaining in neoplasia generally correlates with the presence of a mutation in the coding region of the p53 gene and may provide insights into the pathogenic mechanisms underlying tumourigenesis. p53 immunoreactivity was examined in 77 feline tumours, selected as a representative sample of 486 specimens submitted for diagnosis and analyzed to estimate the relative frequencies of feline neoplasias. Immunocytochemical staining demonstrated nuclear immunopositivity in 46% of the squamous cell carcinomas (SCCs), 50% of the osteosarcomas, 33% of the mammary carcinomas, 16% of the adenocarcinomas and 14% of the haemangiosarcomas. In contrast, none of the malignant lymphomas or fibrosarcomas examined showed p53 immunoreactivity. These data support a role for p53 aberrations in the pathogenesis of certain feline tumours.