Abstract: A vehicle anti-theft system includes a microprocessor and associated radio frequency receiver/transmitter installed within a vehicle. The microprocessor is operably connected to the engine control unit and the existing security system. The vehicle transmitter continuously transmits an interrogating signal within a given transmission field. A remote unit includes a transmitter and a receiver that are in discrete communication with the vehicle receiver and transmitter whenever the remote is within the transmission field. If the remote is outside the transmission field and is therefore unresponsive to the interrogating signal, the microprocessor initiates various security measures such as locking doors, disabling the engine and arming the security system. When the remote carrier reenters the transmission field, the interrogating signal triggers an authorized, encoded response from the remote causing the microprocessor to reverse each of the security measures to prepare the vehicle for routine operation.