In this case series, three unrelated male housemate cats were treated repeatedly with injections of medroxyprogesterone acetate (MPA) for intercat aggression and urinary house soiling. All three cats subsequently developed multiple recurrent mammary adenocarcinomas and underwent numerous surgical resections. This report describes the clinical, histopathological and immunohistochemical findings in these three cats and highlights the potential for mammary carcinomas to develop in male cats years after receiving MPA injections. Extended survival times and a long delay between the administration of the progestin injections and the onset of mammary neoplasia are noted. Estrogen and progesterone receptor staining was performed on some of the tumors and the complex role of hormones in the pathogenesis and the prognosis of feline mammary carcinoma is discussed. Clinicians using MPA should institute life-long surveillance of their feline patients for mammary tumors.