Feline mammary fibroepithelial hyperplasia is a benign but rapid, alarming enlargement of one or more mammary glands that can occur in young intact female cats — sometimes growing to the size of a grapefruit within weeks. It is not cancer, but it can become life-threatening through skin ulceration and infection. Prompt veterinary evaluation and spaying are the solution.
Last reviewed: June 2026
What Is Feline Mammary Fibroepithelial Hyperplasia?
Feline mammary fibroepithelial hyperplasia (FMFH) — also called fibroadenomatous change or benign mammary hypertrophy — is a non-neoplastic, hormone-driven proliferation of both mammary glandular tissue and the surrounding fibrous stroma. It is almost always triggered by progesterone exposure, either endogenous (from the luteal phase of the reproductive cycle) or exogenous (from synthetic progestins such as megestrol acetate or medroxyprogesterone acetate), as described in Côté's Clinical Veterinary Advisor.
FMFH occurs primarily in:
- Young intact females in the first or second estrous cycle (most common group)
- Intact females after mating or pseudopregnancy
- Cats administered synthetic progestins for behavioral or reproductive control
- Rarely, intact or castrated males exposed to exogenous progestins
Unlike true mammary carcinoma (which is hard, irregular, and often ulcerated slowly), FMFH grows extremely rapidly — days to weeks — and the tissue feels soft, doughy, or fluctuant rather than stony hard.
Signs and What the Mammary Glands Look Like
- Rapid, dramatic mammary enlargement — one or several glands enlarge suddenly; the enlargement may be symmetric or asymmetric
- Soft, warm, sometimes fluctuant texture — the tissue feels like water-filled dough, not the stony firmness typical of carcinoma
- Skin thinning and discoloration — the overlying skin becomes taut, shiny, and purple-red from vascular engorgement
- Skin ulceration — when growth outpaces skin elasticity, the skin splits, creating open wounds prone to infection
- Discharge — clear or serosanguineous fluid may leak from ducts or ulcerated areas
- Pain and reluctance to be touched — cats may vocalize or move away when the glands are palpated
- Secondary infection — ulcerated glands rapidly become infected, causing fever, lethargy, and foul-smelling discharge
- Difficulty walking — when very large glands drag against the hind limbs
Diagnosis
Diagnosis is primarily clinical — the history (intact young female, recent progestin administration, rapid growth), the signalment, and the characteristic soft, rapidly enlarging texture are typically sufficient for a presumptive diagnosis.
Your vet may recommend:
- Fine-needle aspirate — confirms non-neoplastic epithelial proliferation; rules out mastitis, mammary carcinoma, or other mass
- Biopsy — definitive histopathology if the clinical picture is atypical or the mass is unusually firm
- CBC and chemistry panel — to assess for secondary infection and baseline organ function before surgery
- Culture of discharge — if ulceration and secondary infection are present
The AAFP-AAHA Feline Life Stage Guidelines, 2021 recommend early ovariohysterectomy in intact females presenting with FMFH.
Treatment
Ovariohysterectomy (spaying) is the primary treatment. Removing the ovaries eliminates endogenous progesterone, and most cases of FMFH begin to regress within 4–6 weeks of surgery. Regression may be incomplete if the tissue has been enlarged for many weeks, but surgical reduction of residual enlarged glands is rarely necessary.
Discontinue exogenous progestins immediately if FMFH is progestin-induced.
Aglepristone (a progesterone receptor blocker available in Europe and some other markets) can induce rapid regression without surgery and is useful when the cat is too systemically compromised for immediate anesthesia.
Wound management for ulcerated glands: gentle cleaning, antimicrobial dressings, and antibiotics (culture-directed) until surgery can be safely performed.
When to See a Vet
Call your vet today if:
- You notice any mammary gland enlargement in your cat — even if it seems soft and painless
- Enlargement has doubled in size over a week
- Skin over the glands looks red, shiny, or thinning
- Your cat is on a progestin medication and mammary changes appear
Go to the ER immediately if:
- Skin has broken down and there is an open, discharging wound over a mammary gland
- Your cat is febrile, lethargic, and not eating with visible mammary changes
- Rapid breathing from pain or systemic infection
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Frequently Asked Questions
Is feline mammary fibroepithelial hyperplasia the same as breast cancer? No. FMFH is a benign, hormone-driven growth that is not cancer and does not metastasize. The distinction matters hugely for prognosis: true feline mammary carcinoma (which requires different treatment and carries a far worse outlook) is hard, irregular, and typically grows more slowly. Both conditions require veterinary evaluation — only a biopsy or aspirate can definitively distinguish them.
Does FMFH go away on its own? It may partly regress after the progesterone stimulus ends — for example, after the heat cycle finishes or progestin is discontinued. However, regression is usually incomplete without spaying, and waiting risks severe skin ulceration, secondary infection, and pain. Spaying is the definitive treatment and should be performed as soon as the cat is stable enough for anesthesia.
Can male cats get mammary fibroepithelial hyperplasia? Yes, but rarely. Intact or castrated male cats exposed to exogenous progestins for behavioral modification can develop FMFH. The treatment is the same: stop the progestin, and the tissue typically regresses. Castration of intact males removes any endogenous androgen that may be converted to progestins in peripheral tissue.
How much does treating FMFH cost? Initial workup — exam, fine-needle aspirate, and bloodwork — runs $150–400. Spaying (ovariohysterectomy) under anesthesia costs $300–700 at most general practices, higher at emergency or specialty centers ($600–1,200). If wound management and antibiotics for ulcerated glands are needed before surgery, add $100–300. Aglepristone, where available, costs $80–200 per injection. Prompt spaying is the most cost-effective path.
At what age is FMFH most common in cats? FMFH most commonly affects young intact females at their first or second estrous cycle — typically 6 months to 2 years of age. This is one of the strongest arguments for spaying cats before their first heat, which also dramatically reduces the lifetime risk of true mammary carcinoma.
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