A pack-off tool comprises a main tubular body including an expandable sealing element located between two non-expandable ring members and being mounted on a mandrel and in sealing engagement therewith at axially spaced locations. The tubular body is axially compressible into a radially extended configuration for sealing engagement with a bore wall. A communicating bore extends through the body between the sealing locations. In use, a high pressure on one side of the tool pushes between the high pressure side of the tubular body and the bore wall, through the communicating bore and into a space between the tubular body and the mandrel, and tends to push the other, lower pressure side of the body outwardly into tighter sealing engagement with the bore wall.