Metabolic alkalosis is the commonest form of acid-base disorder seen in critically ill patients. Although the effects of acidosis have long been known, those of severe metabolic alkalosis are only slowly being recognized. Metabolic alkalosis is itself associated with an increased mortality and a knowledge of the causative factors and treatment options is important. In one study, around 50% of general surgical patients developed postoperative metabolic alkalosis, whereas other acid-base disturbances were uncommon. Metabolic alkalosis results from an accumulation of alkali or a loss of acid. Clinical signs are nonspecific but dehydration may be prominent because of a contraction of the extracellular fluid volume due to loss of chloride. Metabolic alkalosis leads to hypoventilation in patients both with and without lung disease, although in the latter, the effect is relatively transient. In patients with chronic obstructive lung disease, however, the development of metabolic alkalosis leads to prolonged hypoventilation and the establishment of a mixed acid-base disorder that may cause difficulty in weaning in the ventilated patient. This is an often forgotten cause of prolonged stay in the intensive care unit with consequent cost and morbidity implications.