Objective: Companion animals such as dogs share the same environment with people. Mammary tumours are the most prevalent spontaneous neoplasia in intact bitches. More animal models are needed for human mammary tumour research. Our objective was to describe equivalent non-neoplastic and neoplastic mammary gland alterations in woman and dog. Method: Formalin-fixed, paraffin-embedded archived human and canine mammary gland tissue samples. Epidemiological data were collected and analyzed from human (n= 1935) and canine (n=161) patients for the years 2003-2005. Histological comparison was performed with HE-stained tissue sections and immunohistochemistry. Applied antibodies included CK5/6, ERα, PR, Ki67, Her2, p63, SMA, Ecadherin and calponin. Results: Several comparable benign and malignant mammary gland alterations were identified, consisting of lobular, tubular and ductal as well as mesenchymal of origin. Most of these have not been described previously. The epidemiological results show also shared characteristics. Conclusion: Although there are canine specific lesions there also exist equivalent entities. Our results indicate the female canine as a suitable model for translational research.