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Online partial discharge testing has long been used to assess the condition of the high-voltage insulation in motor and generator stator windings. Past research has shown that motor and generator operating parameters such as voltage, power output, gas pressure, and stator winding temperature can have an important impact on the PD levels, and thus must be controlled if useful trends in PD activity over time are to be obtained. Over the years, results have been collected that indicate humidity may also be a factor that needs to be monitored. In addition, some simple laboratory experiments have been performed to verify that humidity can change the PD. When applying online PD testing to assess stator winding insulation condition, users must be aware of the influence of humidity on the measured PD activity to properly interpret the results.