Sodium, potassium and magnesium were measured in serum resp. plasma and in erythrocytes of oliguricanuric children with acute haemolytic uraemic syndrome (HUS). Potassium was in serum resp. plasma and in red cells in normal range or decrease compared with other forms of acute renal failure in children. These findings were different to statements in the relevant literature, which describe hyperkaliaemia. We suppose a potassium depletion in the early state of HUS. The cause is still unknown. Furthermore we found a hyponatriaemia according to severity of edema and gastroenteritis. Sodium in red cells and magnesium in plasma and erythrocytes were slightly elevated.