A hard bag growing inside the buttock flesh is usually caused by perianal cysts, perianal subcutaneous abscesses, folliculitis, lipomas and so on. 1. Perianal cyst: Perianal cyst is a benign tumor lesion, also known as epidermoid cystic mass, which is soft in texture. However, due to the specific nature of the lesion, sedentary behavior may lead to sclerotic changes, manifesting as round or oval shapes. 2. Perianal subcutaneous abscess: Perianal subcutaneous abscess is commonly found in the perianal skin and may be associated with anal fissure, viral infection and lack of attention to hygiene, etc., which leads to inflammatory infection of the perianal skin, resulting in redness, swelling and hardness. The pain will be aggravated when sitting, squatting and defecating. 3. Folliculitis: Buttock folliculitis may be associated with bacterial infections caused by sedentary lifestyle, lack of ventilation and uncleanliness of the buttocks. It appears as a red rash in the early stage, and then may develop into a visible abscess with hard nodules under the stimulation of inflammation, accompanied by itching and pain. 4. Lipoma: Lipoma is usually caused by benign proliferation of mature adipose tissue, fat cells, and can occur in any part of the body with fat, including the femur. It is mainly manifested as subcutaneous swelling with smooth surface, and patients usually have no pain and other feelings. Patients who have a hard bag inside the buttock flesh should go to the hospital in time, under the professional doctor’s examination, to clarify the cause of the disease, and take the relative corresponding treatment.