Nutrition and fluid and electrolyte balance are inextricably linked through ingestion, digestion, absorption and intermediary metabolism. Studies are described showing that man's physiological capacity to excrete excess salt and water is limited under normal conditions; it is further reduced by starvation and injury, so that patients are easily overloaded, resulting in increased complications of illness and surgery. Return of gastrointestinal function postoperatively is delayed by moderate saline overload. Illness not only influences external fluid and electrolyte balance but also internal balance between the extracellular and intracellular spaces and between the intravascular and extravascular components of the extracellular space. The mechanisms and management of these changes are discussed. The importance of fluid and electrolyte balance in nutritional support is emphasised--indeed, the benefits of nutrition may be negated by electrolyte imbalance. The relationships between serum albumin concentration, illness, nutrition and fluid balance are discussed and the limited role of albumin infusions described. Surveys among doctors have revealed a poor knowledge of fluid and electrolyte balance. Measures are needed to improve training.