SummaryPotassium homeostasis is remarkably well preserved in the majority of patients on dialysis, despite the loss of the normal major route of potassium elimination. Nonetheless, the new major pathways for potassium removal (dialysis and gastrointestinal) are clearly less able to adapt to changes in potassium intake than is the healthy kidney. Therefore, disturbances in the plasma potassium concentration, most commonly hyperkalemia, remain a constant threat to the health of dialysis patients. A recent survey of outpatients on chronic hemodialysis found about a 10% incidence of severe predialysis hyperkalemia (serum potassium concentration greater than 6.0 mmol/L). Several causes of this potentially lethal problem must be considered when it occurs (Table 2). We hope that through a sound understanding of potassium homeostasis, the incidence of such serious disturbances in plasma potassium concentration can be reduced.