The article presents a patient who developed severe acute hyponatremia during transurethral prostate gland resection. This case is interesting because of the sudden onset as well as the severity of the initial clinical presentation of hyponatremia, which developed despite meticulous supervision and routine preventive measures. Initial laboratory measurement revealed an extremely low serum sodium level. Combined with the clinical picture (seizures, loss of consciousness), this indicated severe acute hyponatremia, which was then promptly treated. Hyponatremia is the electrolyte disturbance most commonly found in clinical practice and its acute form is seen mostly following transurethral prostate resection. Its other common causes include diuretic use, diarrhea, heart failure, and renal disease. Its clinical manifestations are primarily neurologic, especially in the case of acute hyponatremia, and they mainly include headache, confusion, and stupor, but seizures and coma may also occur. The treatment involves restriction of water intake and promotion of water loss, as well as replacement of any sodium deficit and treatment of the cause.