Massive postoperative polyuria is rare, except in neurosurgery patients. Here we report excessive polyuria in a 59-year-old woman following total gastrectomy for advanced gastric cancer. The etiology of the patient's polyuria was unknown. Urine output was measured hourly and replaced with Ringer's lactate solution at 80% of measured volume. The rate of urine output during 9 postoperative days ranged from 900 to 2700 ml.h(-1). Several administrations of an antidiuretic hormone (ADH) analogue were ineffective in reducing urine output, suggesting a possible relationship of the massive polyuria to nephrogenic diabetes insipidus. Following oral administration of a thiazide diuretic, known to exert an antidiuretic action in nephrogenic diabetes insipidus, urine output was dramatically reduced. We conclude that this case of massive polyuria probably resulted from postoperative nephrogenic diabetes insipidus.