Abstract: A hydraulic percussion tool for developing repeated impact blows for rock drilling and other high force applications is characterized by a piston hammer reciprocably disposed in a cylinder bore and a tubular valve member coaxial with and actuated by the hammer for controlling the flow of pressurized hydraulic fluid to and from portions of the cylinder bore. The valve member is actuated by the hammer on the return stroke thereof through a column of fluid trapped in an annular chamber formed between a transverse surface on the hammer and an end face of the valve member. Valving mechanism is operable to effect controlled venting of the trapped fluid from the annular chamber to change the position of the valve member with respect to the hammer and thereby control the hammer stroke length, impact blow, and blow frequency.