Boils that leave a hard lump after healing are usually caused by infection, but also considered to be related to scarring and other factors, usually can be treated with surgery. 1. Infection: boils are caused by bacterial infection of the hair follicle, and after healing due to the stimulation of infection and inflammation, the local fibrous hyperplasia may leave a hard lump. This situation generally does not need treatment, or infrared physical therapy, after about six months to a year, inflammatory substances gradually subside, absorption, hard lumps can be gradually eliminated. 2. Scarring: If the boil is seriously infected, hard lumps may appear after healing due to scarring. It can be treated with medication such as Halcionide solution and silicone gel under doctor’s supervision; physical therapy by infrared radiation; or surgical excision if necessary. If a boil heals and leaves a hard lump, go to the hospital promptly.