The importance of obtaining a careful history of the onset of the illness leading to the electrolyte upset cannot be over emphasised. Most electrolyte disturbances which can be corrected by intravenous therapy arise as a result of either abnormal losses of body fluids or of deficient replacement of normal losses. The history often reveals the cause of the illness and provides important clues to the type of biochemical abnormality present. The laboratory results should serve merely as confirmation and as a guide to the degree of abnormality to be corrected. Indeed, if the laboratory results are not in keeping with the clinical history, the laboratory tests should be repeated in case a laboratory error has occurred.