The invention concerns a method and a system for controlling start-up of a motor vehicle by dialogue of the engine controlling computer with the piloted mechanical gearbox, during a restart of the vehicle in stop and start mode. A “start-up authorization” is transmitted to the gearbox at a time t when the following three conditions are fulfilled: a) verifying the position of the accelerator pedal relative to a predetermined “threshold” position; b) the engine is in sequential injection phase; c) verifying a so-called “autonomous condition”, calculated on the basis of the engine speed and of an engine speed gradient compared to a predetermined threshold value. The invention is applicable to motor vehicles.