A hydraulic system for a vehicle, comprising at least one hydraulic, load-carrying assembly which includes at least one movable structural element and at least one hydraulic device for actuating the structural element, said hydraulic system comprising a primary circuit which includes a servo device, a stationary hydraulic pump driven by a motor, a directional valve, and said at least one hydraulic device, said directional valve being arranged between the hydraulic pump and the hydraulic device for allowing hydraulic oil to flow to the hydraulic device at a servo signal from the servo device in order to operate said assembly, wherein the hydraulic system comprises a supplementary circuit which is connected to the primary circuit and includes a variable hydraulic pump which is driven by the motor and which is arranged for supplying an adjustable addition of hydraulic oil to the hydraulic device, and a proportional valve which is arranged between the variable hydraulic pump and the hydraulic device in order to regulate the flow of hydraulic oil to the hydraulic device as a function of a received flow signal, wherein said variable hydraulic pump includes a load-detecting regulator which is arranged for detecting the load on the hydraulic device when the variable hydraulic pump is in operation, characterized in that - the supplementary circuit includes an electronic control unit, which is connected to the primary circuit through a first sensor which is arranged at the servo device in order to detect said servo signal and to transmit an electric control signal to the control unit, said control signal being a function of the servo signal, as well as through a second sensor which is arranged at the hydraulic device in order to detect the load on the same and to transmit an electric load signal to the control unit, said load signal being a function of the load on the assembly, - the proportional valve is connected to the control unit in order to receive said flow signal in the form of an electric signal therefrom, said flow signal being a function of the control signal and the load signal, - the supplementary circuit includes a throttle pedal and a third sensor, said third sensor detecting the position of the throttle pedal and transmitting an electric throttle-regulating signal to the control unit, and that - the control unit is arranged for transmitting an electric speed signal to an adjusting member which is arranged at the motor in order to regulate the motor speed, said speed signal being a function of the throttle-regulating signal, the control signal and the load signal. - the supplementary circuit includes an electronic control unit, which is connected to the primary circuit through a first sensor which is arranged at the servo device in order to detect said servo signal and to transmit an electric control signal to the control unit, said control signal being a function of the servo signal, as well as through a second sensor which is arranged at the hydraulic device in order to detect the load on the same and to transmit an electric load signal to the control unit, said load signal being a function of the load on the assembly, - the proportional valve is connected to the control unit in order to receive said flow signal in the form of an electric signal therefrom, said flow signal being a function of the control signal and the load signal, - the supplementary circuit includes a throttle pedal and a third sensor, said third sensor detecting the position of the throttle pedal and transmitting an electric throttle-regulating signal to the control unit, and that - the control unit is arranged for transmitting an electric speed signal to an adjusting member which is arranged at the motor in order to regulate the motor speed, said speed signal being a function of the throttle-regulating signal, the control signal and the load signal.