Water jet surgical instruments comprising a feed line for supplying fluid in one flow direction and one exit nozzle for ejecting the fluid in the form of a fine jet displaying a defined geometric opening configuration are known. In order to be able to widen the jet a fluid chamber is provided—viewed in the direction of flow—upstream of the exit nozzle. A disturbance device is provided in or on the fluid chamber, said disturbance device being able to generate a turbulent flow inside the fluid chamber.