Disclosed is a method for assessing deterioration in a gearbox. Data is obtained from an operating gearbox regarding harmonic frequencies and their related sidebands generated as a consequence of the gearbox operation. In a first embodiment, a Sideband Energy Ratio (SER) is calculated by dividing the amplitude of the sum of sideband signals associated with a harmonic to the amplitude of the harmonic. In a second embodiment, a SER is calculated based on sideband amplitudes associated with adjacent harmonics. The ratio may be monitored over time or compared to one or more values to provide an indication of deterioration.