Oedematous malnutrition, represented by its most severe form kwashiorkor, is rampant in many parts of the world and is associated with a high case fatality rate. Despite being first described more than a century ago, the pathogenesis of kwashiorkor is still not clear. The traditional thinking is that it results from a deficiency of dietary protein and is usually associated with an infection. This has now been challenged by the finding that there is no difference in diets of children developing marasmus or kwashiorkor. Nutritional oedema is associated with an increased secretion of anti-diuretic substance (probably antidiuretic hormone) which prevents the normal excretory response to water administration. Experimental studies have shown that feeding low-protein, low-calorie diets results in delayed and incomplete response to a water load, and that the livers of the animals show a reduced capacity for inactivating anti-diuretic hormone. There is now evidence that links generation of free radicals and depletion of anti-oxidants with the development of oedema in kwashiorkor.