Gill slit cysts may continue to increase in size if left untreated, and gill slit fistula can occur if rupture or infection occurs, as follows. 1. Gill slit cysts are mostly manifested as painless lumps on the lateral side of the face and neck, which are slow-growing, with smooth surface, soft texture, clear boundaries and movable. Puncture examination reveals clear, cholesterol-containing or non-cholesterol-containing cystic fluid. 2. If left untreated, gill slit cysts may continue to grow in size, affecting normal life and work. In the event of rupture of the gill slit cyst or upper respiratory tract infection, a gill slit fistula may form, and the area of the cyst may be red, swollen and painful. If the above symptoms occur, it is recommended to go to the hospital in time, with the help of the doctor for early targeted treatment.