Destruction of diseased tissue in situ by means of freezing is well established in many branches of surgery. The tissues are apparently unaltered at thaw but progressive necrosis ensues. There is controversy as to whether tissue death is principally due to the direct effects of freezing or to subsequent ischaemia. Studies at the ultrastructural level show that ice-crystals are formed within the cells during cryosurgery, that resultant cell damage is osmotic rather than mechanical and that microcirculatory changes are secondary in terms of the chronological development of tissue necrosis.