Pseudohyperkalemia, defined as serum to plasma potassium difference of more than 0.4 mmol/l, occurs when platelets, leukocytes or erythrocytes release intracellular potassium in vitro, leading to falsely elevated serum values. Pseudohyperkalemia has been observed in myeloproliferative disorders [ 1 ], including leukemia [2] and infectious mononucleosis [3] as well as in rheumatoid arthritis [4]. We present 2 patients with renal disease and thrombocytosis in whom pseudohyperkalemia was recognized only after common therapeutic measures and/or dialysis failed to effect a decrease in serum potassium. In patients with renal disease and thrombocytosis, plasma as well as serum potassium should be routinely measured prior to instituting aggressive therapy or altering dialysis prescription in order to avoid potentially dangerous overtreatment with resulting hypokalemia.