Various polyethylene foams were subjected to thermo-mechanical processing with the aim of transforming them into re-entrant materials exhibiting negative Poisson's ratio. Following transformation, large cell foams (cell sizes of 1 and 2 mm) exhibited re-entrant cell structure and negative Poisson's ratio over a range of processing times and temperatures. Poisson's ratio vs. strain for these foams was similar to prior results for reticulated polyurethane foams. Following processing, microcellular polyethylene foam was densified but cells remained convex; it did not exhibit a substantial negative Poisson's ratio. This foam had a different transition temperature as determined via DSC than the large cell foams.