In the present case-control study several dietary and nutritional factors were investigated to determine if a relationship exists between diet and development of mammary tumors in female dogs. Control female dogs (n = 86) were compared with a case group of dogs (n = 102) with dysplasias or tumors of the mammary gland. A questionnaire providing information on the dog's body conformation and dietary and reproductive histories was answered by the owners. Serum selenium and retinol concentrations and the fatty acid profile in subcutaneous adipose tissue were analyzed as indicators of nutritional status. Obesity at 1 year of age and 1 year before the diagnosis of mammary nodules was found to be significantly related to a higher prevalence of mammary tumors and dysplasias. The intake of homemade meals (compared to that of commercial foods) was also significantly related to a higher incidence of tumors and dysplasias. Other significant risk factors were a high intake of red meat, especially beef and pork, and a low intake of chicken. The subcutaneous fatty acid profile and the serum selenium concentration were not significantly different in the cases and the controls, with the exception of C18:1 fatty acid (oleic acid) content, which was significantly higher in the cases than in healthy controls. Serum retinol concentration was significantly lower in the cases than in the controls. In the multivariate analysis, older age, obesity at 1 year of age, and a high red meat intake were independently and significantly associated with the risk of developing mammary tumor and dysplasias.