Objective. To study the prognostic value of disturbances of the water and electrolyte homeostasis in the acute stage of ischemic stroke (II) and their impact on the course and outcome. Material and methods. Disturbances of the water and electrolyte homeostasis associated with plasma sodium fluctuations were studied in 150 patients with severe II. Results. The poor outcome was associated with plasma osmolarity >297 mOsmol/L and plasma sodium concentration >155 mOsmol/L in the first day of severe II and with >303 mOsmol/L and >161 mOsmol/L, respectively, in the 3rd and 5th days. The prognosis was significantly worse in hypovolemia compared to normo- and hypervolemia. Conclusion. Hypovolemic hyponatremia as a presentation of hyperosmolar syndrome at the onset of severe II can be considered as a relatedly independent predictor of the fatal outcome.