An hydraulic heel-height adjustment device for a lower limb prosthesis has a foot-mounting component (18) for detachably mounting a prosthetic foot and a shin component pivotally connected to the foot-mounting component allowing angular adjustment of the foot-mounting component relative to the shin component about a heel-height adjustment axis. The relative angular position is set by an hydraulic piston (42) and cylinder assembly which has a chamber (40) in the shin component containing a piston (42) arranged to execute a translational movement in the chamber when adjustment is required. A bypass passage interconnects parts of the chamber on opposite sides of the piston, this passage containing a manually operable valve which is spring-biased towards a closed position. The piston has a piston rod pivotally connected to the foot-mounting component by a pivot joint defining a medial-lateral axis parallel to and spaced from the heel-height adjustment axis.