In this study with randomized controls, we administered fludrocortisone acetate to 46 of 91 patients with subarachnoid hemorrhage in an attempt to prevent excessive natriuresis and plasma volume depletion. Fludrocortisone significantly reduced the frequency of a negative sodium balance during the first 6 days (from 63% to 38%, p = 0.041). A negative sodium balance was significantly correlated with decreased plasma volume during both the first 6 days (p = 0.014) and during the entire 12-day study period (p = 0.004). Although fludrocortisone treatment tended to diminish the decrease in plasma volume, the difference was not significant (p = 0.188). More patients in the control group developed cerebral ischemia (31% vs. 22%) and, consequently, more control patients were treated with plasma volume expanders (24% vs. 15%), which may have masked the effects of fludrocortisone on plasma volume. Fludrocortisone therefore reduces natriuresis and remains of possible therapeutic benefit in the prevention of delayed cerebral ischemia after aneurysmal subarachnoid hemorrhage.