Blood samples were obtained from a radial artery cannula in a 60-yr-old man during coronary artery surgery. Serum potassium concentrations of 9.3, 8.4, and 7.4 mmol litre(-1) were obtained. A simultaneous venous blood sample gave a serum potassium concentration of 4.4 mmol litre(-1). The ECG was unchanged. After the arterial cannula was repositioned, subsequent blood samples gave expected ranges of serum potassium concentration. We suggest that the initial arterial cannula position caused a high shear rate in the blood when samples were withdrawn, causing haemolysis and hyperkalaemia.