Feline mammary gland hyperplasia is a disease that consists of a localized or diffuse enlargement of one or more mammary glands and it affects young females both cycling or pregnant queens. Cats of either gender that are receiving exogenous progestogen are prone to develop the problem. It is believed that the problem develops as a result of an increase in the number of progestogens receptors within the mammary glands as only a third of the affected cats show elevated serum concentrations of the sexual hormones. Signs generally appear in an acute manner and the affected mammary gland (or glands) are generally edematous, erythematous and in some cases this can lead to skin ulcerations. Diagnosis is based on the clinical history and cytology examination. Differential diagnosis are mastitis and mammary neoplasia. If reproductive interest is not an issue, ovariohysterectomy should be considered. Alternatively, the use of progesterone receptor blockers such as aglepristone, prostaglandin F2α, testosterone or prolactin inhibitors (bromocriptine). Mastectomy is only recommended in cases of unsuccessful results following medication or in cases which skin ulcerations are present. The present report refers to a case of a 1 year old entire female crossed bred cat that was brought to the clinic in October 2008. The cat showed a diffuse edema of all mammary glands. According to the owner, the signs had fourteen days progression with a marked increase of the enlargement in the past three days. The owner never used progestagen in the cat. It was then recommended prednisone and enrofloxacin for seven days. After two days the owner brought the cat back to the clinic reporting a worsening of the problem. Treatment with prednisone and enrofloxacin was continued and ovariohysterectomy was performed at the ninth day following the initial consultation. There was then a decrease of the enlargement at thirteenth day but at the forty-first day the mammary enlargement returned with a floating aspect at this time. At the forty-eighth day we introduced treatment with aglepristone and this was followed by a 30% decrease in the volume of the mammary glands in the sixth application. Improvement kept progressing and at the seventieth-fifth day there was only a subtle edema limited to M2 e M5. Contrary to what was expected, neutering did not result in success in this case. However, aglepristone was effective showing about 90% improvement within forty days from the first application.