SummaryPotassium deficit constitutes a distinct clinical entity which must be differentiated from hypokalemia. It is characterised by the insidious development of digestive, cardiac, renal and neuromuscular abnormalities. There are but few other fields in physiopathology where clinical data have so nicely confirmed the facts observed in the laboratory by physiologists and biochemists.The aetiology of potassium depletion is protean. This electrolyte imbalance may supervene in the course of renal, surgical or digestive disease. Since the introduction of gastric suction, adrenocortical hormones, and new oral diuretic drugs in therapy, potassium deficiency has definitely joined the growing list of iatrogen diseases.This often dramatic complication of illness or of its treatment will be avoided if the physician disposes of laboratory facilities to obtain a prompt determination of the electrolyte concentrations in plasma and in urine.