Pseudohyperkalemia occurs occasionally in patients with extreme leukocytosis. Increased white blood cell fragility coupled with mechanical stress is felt to be causal. Serum and plasma potassium levels have been both associated with pseudohyperkalemia. Whole blood potassium determination will usually verify the correct diagnosis. It is important to diagnose this condition early so that patients are not inappropriately treated. Two patients with chronic lymphocytic leukemia and extreme leukocytosis are presented, one with pseudohyperkalemia and one with probable pseudohyperkalemia, and diagnostic considerations are discussed.