Nutritional therapy can be impaired if imbalances in water and electrolyte status have led to gross disorders of the cardiovascular, pulmonary, renal, metabolic, and central nervous systems. Restauration and maintenance of the functional extracellular fluid volume is the primary therapeutic goal in water and electrolyte resuscitation. Hyper- and hypoosmolar disturbances are automatically corrected by intrinsic regulatory mechanisms. Potassium deficiency or overload, or potassium disequilibrium between the intracellular and extracellular space can lead to dangerous cardiac arrhythmias. Hyper- and hypokalemia usually develop within days or even weeks and should not be corrected within a few hours. If life threatening hyperkalemia develops during acute renal failure, 20 ml 10% calcium gluconate solution can be given intravenously in order to avoid ventricular fibrillation or cardiac arrest. The discrimination between prerenal disease, acute tubular necrosis and other causes of acute renal failure is based on special investigations, such as urinary osmolality, urinary sodium concentration, clearance of creatinine, osmolar solutes, free water, and fractional sodium excretion. The clinical examination of a patient should be the basis of assessing his water and electrolyte state. Laboratory findings which are in disagreement with the clinical state have to be repeated, critically interpreted, but not completely rejected. Third space losses make fluid balance difficult.