Internal discharge and electrical treeing in polymers involve the presence of voids or the creation of low-density channels at high fields to provide a favourable condition for impact ionization. A great deal of experimental evidence indicates that polyethylene contains a high density of microvoids with the sizes varying from less than 1 µm to larger than 10 µm. The microvoids may not be the direct cause of electric discharge, but they may be the source to initiate the formation of low-density channels which in turn initiate the formation of trees. The long-term deterioration in polymers is generally associated with the formation of trees. The purpose of this paper is to present some new results in support of the model that the creation of low-density channels under high fields is the prelude of the formation of trees based on the pressure dependence of electric discharge and treeing properties in polyethylene.