Mammary carcinoma in cats is a biologically aggressive cancer arising from the milk-producing glands. A critical difference from dogs is that 85–90% of feline mammary tumours are malignant — far higher than the roughly 50% rate in dogs. It is the third most common cancer in cats (after lymphoma and skin tumours) and most frequently affects older, intact (unspayed) females. As in dogs, the risk is dramatically reduced by early ovariohysterectomy. Tumour size is the single most important prognostic factor: tumours smaller than 2 cm carry a significantly better prognosis than larger ones. The cancer tends to spread to the lungs and regional lymph nodes. Siamese cats are overrepresented and may develop the disease at a younger age. Surgery — ideally radical mastectomy (often performed as a staged bilateral procedure) — is the mainstay of treatment, with chemotherapy considered for larger or more advanced tumours.