In a logical, stepwise approach to patients presenting with hypokalaemia or hyperkalaemia the clinician must first recognise circumstances in which the dyskalaemia represents a clinical emergency because therapy then takes precedence over diagnosis. If a dyskalaemia has been present for a long time, there is an abnormal renal handling of K+. The next step to analyse is the rate of excretion of K+ and, if necessary, its two components (urine flow rate and K+ concentration in the cortical collecting duct [CCD]) analysed independently. If the K+ concentration in the CCD is not in the expected range, its basis should be defined at the ion-channel level in the CCD from clinical information that can be used to deduce the relative rates of reabsorption of Na+ and Cl- in the CCD. This analysis provides the basis for diagnosis and may indicate where non-emergency therapy should then be directed.