Abstract: A power-assisted steering system for a vehicle comprising a hydraulic motor (1) for providing vehicle steering assistance, an open center servo-valve (6) for supplying the hydraulic motor (1) with hydraulic fluid, a hydraulic pump (7) for supplying hydraulic fluid to the servo-valve (6) and to the hydraulic motor (1), and an electric motor (13) for driving the hydraulic pump (7). The hydraulic pump (7) is controlled by controlling electrical power consumed by the electric motor (13). At least one sensor (15) repetitively monitors at least one operating parameter of the power-assisted steering system. At least one microprocessor (17) repetitively calculates reference values that vary as a function of a monitored level of the at least one operating parameter.