A longitudinally adjustable connecting rod for a piston engine, where the connecting rod includes a first connecting rod end for receiving a piston pin and a second connecting rod end for receiving a crankshaft journal, where the distance between the piston pin and the crankshaft journal is adjustable in the longitudinal direction (A) of the connecting rod by way of a hydraulic control device with a hydraulic control valve. The hydraulic control valve comprises a control cylinder and a control slide which is guided in a slidable manner in the control cylinder and which can be pressurized, and at least one outlet valve which can be actuated by the control slide. The control slide comprises a control piston which is arranged on the end face, with a control pressure surface which can be subjected to the hydraulic control pressure and which defines a control pressure chamber in the control cylinder. The use of such a longitudinally adjustable connecting rod with a hydraulic control device in a piston engine as well as a corresponding piston engine is also provided.