Objective : Subarachnoid hemorrhage[SAH] is commonly associated with polyuria [solute diuresis or water diuresis]. The authors investigate the incidence and clinical characteristics of polyuria with special reference to the administration of osmotic diuretics. Methods : One hundred and forty eight patients with high urine output [>200ml/hr] after ruptured cerebral aneurysm operated early from Jan 1998 to Jun 2003 were selected. Water diuresis [diabetes insipidus, DI] was differentiated from solute diuresis by lower urine specific gravity [<1.005] and higher plasma osmolality. The incidence and mode of onset of polyuria were compared between two types of diuresis. Additionally, the relationships between development of polyuria and clinical features including aneurysm location, clinical grade, Fisher grade, and outcome were analyzed. Osmotic diuretics were not routinely used in patients with Hunt-Hess grade I-III since July 2001. Results : Annual incidence of polyuria decreased markedly since July 2001 : 45.2% in 1998, 34.5% in 2001, 11.9% in 2003. Postoperative DI occurred in $2.4{sim}11.1%$. DI developed mainly from ruptured anterior communicating artery aneurysm. The mean interval between the last SAH and the onset of DI was 7.1 days [range $1{sim}27$ days] and lasted mean 4.6 days. When compared with solute diuresis, the development of DI was significantly delayed. Other clinical features were not closely related to polyuria. Conclusion : Uncontrolled polyuria may lead to cerebral ischemia and electrolyte imbalance because SAH patients are already predisposed to hypovolemia, and will risk precipitating the opposite situation with overhydration. We can decrease the development of polyuria without routine use of osmotic diuretics, by avoiding the increased intracranial pressure such as the intraoperative ventriculostomy and gentle brain retraction in good grade patients.