In two patients in the blast phase of chronic myeloid leukaemia there was a marked rise in serum potassium levels, while plasma potassium was at the lower range of normal and there were no signs of hyperkalaemia clinically. After reduction of the pathological cells under cytostatic treatment the serum potassium levels returned to normal. This in-vitro phenomenon seems to be more common in patients with marked thrombocytosis and leukaemia than has previously been thought. In such patients hypokalaemia may be undetected because serum potassium levels arenormal. Pseudo-hyperkalaemia exists when the potassium level is normal in plasma obtained by centrifugation immediately after the blood sample has been taken, without addition of plastic spheres and at once separated from the blood cells.