Phosphatase and tensin homolog (PTEN) belong to the group of gatekeepers tumour suppressor genes and is involved in multiple mechanisms leading to cellular defense against neoplastic transformation and progression. PTEN expression in feline and canine mammary gland tissues was investigated setting up an immunohistochemical method. Twenty four dogs and 17 cats were submitted to a two-years follow-up and clinico-pathological features were recorded and compared with immunohistochemical PTEN expression. PTEN-negative status occurred in 33% of canine and 76% of feline mammary carcinomas. In canine mammary carcinomas there was a significant (P<0.05) correlation among PTEN loss of protein expression and simple carcinoma histotype, presence of lymphatic invasion, lymph-node metastases, distant organ metastases, tumour dedifferentiation, tumour recurrence, and shorter overall survival. In feline mammary tumours a significant correlation between PTEN protein expression and presence of lymphatic invasion was found. Loss of PTEN expression could be a useful prognostic marker in canine and perhaps feline mammary carcinomas.