Hyperkalemia is a life-threatening condition potentially leading to cardiac arrest. Here, we report a case of surprising severe hyperkalemia of 10.2 mmol/L in a diabetic patient with previously normal kidney function presenting without discernible clinical symptoms to our emergency department. The patient was admitted because of hyperglycemia of 32.8 mmol/L, which was detected during daily testing in her nursing home. The hyperkalemia was caused by prerenal failure due to hyperglycemic polyuria which led to volume depletion, and worsened by a combination of potassium-sparing drugs and potassium supplementation. The patient was treated conservatively. Eighteen hours later, the serum potassium concentration was 4.6 mmol/L. The patient could be released 6 days later. To our knowledge, this is the highest described hyperkalemia treated conservatively and survived without cardiopulmonary resuscitation.