Background Cryohydrocytosis (CHC) is an inherited dominant haemolytic anaemia characterised by mutations in a transmembrane segment of the anion exchanger (band 3 protein). Transfection experiments performed in Xenopus oocytes suggested that these mutations may convert the anion exchanger into a non-selective cation channel. The present study was performed to characterise so far unexplored ion transport pathways that may render erythrocytes of a single CHC patient cation-leaky. DESIGN AND METHODS: Cold-induced changes in cell volume were monitored using ektacytometry and density gradient centrifugation. Kinetics, temperature and inhibitor-dependence of the cation and water movements in the CHC patient's erythrocytes were studied using radioactive tracers and flame photometry. Response of the membrane potential of the patient's erythrocyte membrane to the presence of ionophores and blockers of anion and cation channels was assessed. RESULTS: Swelling in CHC patient's erythrocytes in the cold was observed in KCl-containing, but not in NaCl-containing or KNO(3)-containig media indicating that volume changes were mediated by an anion-coupled cation transporter. In NaCl-containing medium the net HOE-642-sensitive Na(+)/K(+) exchange prevailed, whereas in KCl-containing medium swelling was mediated by a chloride-dependent K(+) uptake. Unidirectional K(+) influx measurements showed that the patient's cells have abnormally high activities of the cation-proton exchanger and the K(+),Cl(-)-cotransporter, which can account for the observed net movements of cations. Finally, both chloride and cation conductance in the patient's erythrocytes did not differ from that of healthy donors. Conclusions These results suggest that a cross-talk between the mutated band 3 and other transporters might increase the cation permeability in cryohydrocytosis.