If your Thermophore® is an older unit (purchased prior to 2005) the cover is 100% cotton. It should be washed in cold water and allowed to air dry, in order to avoid shrinking. If you have purchased a new Thermophore®, the cover is a cotton/poly blend which can be machine washed and dried. For best performance, do not over-dry the cover, since it will take some time for an extremely dry cover to regenerate any moisture. The inner fabric cover of the heating unit itself is not washable.