Abstract: A hydraulically operated mechanism which utilizes engine oil as the hydraulic fluid to open and close a cylinder valve is easily installed on a conventional internal combustion engine to improve engine performance over a wide range of operating parameters. A primary assembly has a piston that is actuated by the rotational movement of the cam lobe to pressurize oil inside a tube or hose and deliver pressurized oil to a secondary assembly to push open the valve. The secondary assembly has a secondary piston that utilizes a spring to abut against the valve. The hydraulic variable valve control mechanism optimizes the engine valve event by being self-adjusting and by virtually eliminating valve lash, resulting in improved engine performance, including horsepower, fuel consumption and exhaust emissions, at all ranges of engine speed.