A size-adjustable seton (1) for treating fistulae (32) is provided with first and second wire ends (4, 6), which are provided with connecting means (8, 10) for, in a connected state, forming a smooth connection (12); in such a way, the seton (1) is a smooth, closed loop (3). The invention also relates to a method of forming the closed loop using the aforesaid seton (1), and a melt clamp (40) for use when placing the seton (1) in a person (30).