Butt crack grows a bag very painful in most cases is due to perianal abscess, thrombosed external hemorrhoids, sebaceous cysts or lipoma with infection and other reasons. 1. Perianal abscess: mainly manifested as a sudden increase in perianal mass, mass and its surrounding red and swollen, pressure pain is obvious, and pus flow when broken. 2. thrombosed external hemorrhoids: the patient has a history of hemorrhoids, manifested as hemorrhoidal nuclei protruding after the inability to retract, the lump was dark red, the lump ruptured after the pain improved. 3. sebaceous cysts or lipomas with infection: patients have a history of painless lumps, lumps grow slowly, no discomfort, there is an infection, the lumps at the increase in size with obvious redness and swelling. Butt crack long bag is also seen in other reasons, should be timely consultation, under the guidance of professional doctors standardized treatment, so as not to delay treatment.