A patient undergoing transurethral resection of the prostate suffered hyponatremia during the perioperative period. Electroencephalography demonstrated diffuse slowing although the patient was not clinically encephalopathic. The serum sodium level may indicate a trend toward development of the transurethral resection syndrome but the sodium level does not necessarily parallel metabolic changes in brain tissue. Consequently, electroencephalography may aid in the early diagnosis and treatment of encephalopathy during the early phases of the transurethral resection syndrome.