OBJECTIVE : Hyponatremia has been frequently observed after aneurysmal subarachnoid hemorrhage (SAH), and some data have suggested a correlation with symptomatic cerebral vasospasm and poor outcomes. The present prospective study investigated sodium and water disturbances after aneurysmal SAH with regard to symptomatic vasospasm and patient outcomes. METHODS : Data from all patients with aneurysmal SAH treated in our department during a 2-year period were collected. Daily natriuresis, sodium levels, water balance, and serum and urine osmolality were measured at 4 different points: day 1 of admission or bleeding, day 3, day 7, and day 14-21 or discharge. The clinical parameters (i.e., Hunt and Hess grade, aneurysm location and treatment, onset of vasospasm) were reviewed. The patients' outcome was assessed using the Glasgow outcome score and modified Rankin scale. RESULTS : A total of 101 patients (70 women; median age, 52 years) were enrolled in the present study. Of these 101 patients, 59.4% had a good grade SAH (Hunt and Hess grade 1-3). The most common aneurysm location was the anterior communicating artery (37%). The results from an electrolyte analysis were available for ≤91 patients at days 1 and 78 at discharge. In 33 patients (32.7%), hyponatremia had been diagnosed at any time point. Hyponatremia was most frequently observed at day 1 and later at days 7-10. A location in the anterior communicating artery resulted in hyponatremia more frequently only at day 1 (P = 0.007). The main causes of hyponatremia were cerebral salt-wasting syndrome (early onset) and syndrome of inappropriate antidiuretic hormone secretion (early and late onset). CONCLUSIONS : Distinguishing early- and late-onset hyponatremia is of major relevance, because different therapeutic approaches are required. Only hyponatremia at discharge resulted in less favorable outcomes.