A new method of measuring the change in the average height above ground of horizontal high-voltage overhead transmission-lines (OHTLs) phase conductors is introduced. The new technique, called power-line carrier sag (PLC-SAG) for short, determines average overhead conductor height variations in real time by correlating sag with measured variations in the amplitude of signals propagating between power-line carrier (PLC) stations. The multifrequency PLC-SAG monitoring signals are injected onto the operational PLC system in the 50- to 500-kHz band, but without interfering with the operational integrity of the PLC teleprotection system. The feasibility of the method was examined using the theory of natural modes for multiconductor systems, and tested by extensive field experiments on two horizontal 400-kV lines operated in South Africa by Eskom. It is concluded that the average height of a horizontal OHTL can be tracked accurately for continuous periods by the PLC-SAG technique.