A hard lump under the armpit that is not painful and does not move is more likely to be a parametrial breast, which is a congenital developmental abnormality. If a female patient is pregnant or has given birth, the parametrial breast may increase in size and become more visible. Some patients with parametrial breast disease may also have nipples, or in severe cases, they may have hyperplasia with regular painful symptoms. Ultrasound can also show a localized mass that resembles a mammary gland with a tender texture on palpation. Although parametrial disease is a benign disease, it should be treated with surgery at an early stage after diagnosis because it affects the aesthetics and has the potential to become malignant. The surgery can be performed with a small axillary incision and cosmetic suturing method to free the lesion completely and precisely, avoiding the glandular residue of the parametrium, which can lead to recurrence after surgery.