Background Despite its high demand, prostheses are too often abandoned because they fail to meet the main requirements control, comfort and cosmesis. Body-powered hook prostheses provide good control due to their proprioceptive feedback but are in general not efficient nor appealing. The WILMER group has developed an appealing voluntary-opening prehensor by building all mechanical parts inside the prosthetic cover. The voluntary closing (VC) version, the Tweezer, has slight advantages over the voluntary opening but failed to provide sufficient energy transmission within the available space to which hydraulics offer the solution. The objective is to develop and evaluate a hydraulic VC mechanism within a body powered hook prosthesis that meets the main requirements. Methods A hydraulic system was designed. The system shall transmit a 32 N and a 6.4 N actuation force, when holding objects for a prolonged time, to a 7 N pinch force. A maximum 40 mm actuation displacement shall close the hook from a 50° open to a 0° closed position. The system shall fit inside the prosthetic cover and have a maximum mass of 43 g. A prototype was built and tested to measure its performance. Results The design consists of three hydraulic actuators: a master cylinder, a slave cylinder and a pressure intensifier. The user actuates the master cylinder, which is incorporated in the harness, causing fluid to be transferred to a slave cylinder in the prosthesis that closes the hook to grasp objects. When an object is met the pressure intensifier amplifies the pressure of the slave to increase the pinch force to 45 – 53 N and 9 – 11 N for actuation forces 32 and 6.4 N respectively. The actuation displacement equals 26 mm. The prototype failed to be manufactured properly within the given time and a simplified version, equal to the inactive pressure intensifier, has been made to prove part of the concept. Theoretically, the simplified version closes the hook with an actuation displacement of 11 mm and reaches pinch forces of 5 – 6 N over the hook range. The prototype closes the hook with 12 mm and reaches pinch forces 3 – 5 N. It takes 4 N to reopen the hook. The simplified version weighs only 15 g and fits easily inside the prosthetic cover. The mechanism provides proprioceptive feedback at the master cylinder. Conclusion Part of the concept has been proven. Theoretically, by adjusting the simplified versions parameters the 32 N actuation force could generate pinch forces 16 – 19 N with a 34 mm actuation displacement. It would outperform other body powered prostheses in terms of force transmission. The hydraulic VC mechanism has the potential to transfer a comfortable actuation force and displacement into the required pinch force within the available space of the Tweezer. Thus, a voluntary closing hook prosthesis is designed that can satisfy all user requirements control, comfort and cosmesis.