We describe two families with the 'cryohydrocytosis' form of stomatocytosis. Both show a mild stomatocytic anaemia with Hb levels of 12-16 g/dl and reticulocyte counts of 4.3-24%, with very marked autohaemolysis at refrigerator temperatures and pseudohyperkalaemia as a result of loss of K from red cells on storage at room temperature. The ouabain + bumetanide-insensitive 'passive leak' K influx showed a 'U'-shaped temperature dependence, with a minimum at 23 degrees C. In one family, there was consistent variation in haematological severity within the pedigree. In the other, the parents of the proposita were normal, but all three of her children were affected, consistent with a new mutation of a dominant condition. Cold storage of the red cells led to a very marked increase in osmotic fragility and macrospherocytosis, explaining why a diagnosis of 'hereditary spherocytosis' can easily be reached in these pedigrees.