Eighty-eight cases with water and electrolyte disturbances, including polyuria, hyper- or hyponatremia, were found in 1,000 cases of surgically treated cerebral aneurysms and 80 nonoperated cases. In this paper, the clinical courses of the 88 cases were studied and an investigation was made of the hypothalamic lesions seen in ten autopsy cases. Water and electrolyte disturbances were most numerous in cases of anterior communicating aneurysms, and the prognosis was poor. Those with hypernatremia had a poor prognosis, with a 42% mortality rate during hospitalization. In contrast, the mortality rate for those with hyponatremia was 15%. Post-mortem studies showed various hemorrhagic and/or ischemic changes in the hypothalamus, with a high incidence of cerebral vasospasm. Massive hemorrhages in the hypothalamus tended to be associated with hypernatremia.