Phosphorus depletion in malnutrition has not generally received attention. Serum phosphorus was measured in healthy infants (1.8 mmol/L), in well-nourished infants with acute dehydrating gastroenteritis, and in infants suffering from malnutrition. Serum phosphorus levels were found to be low in well-nourished infants with acute dehydrating gastroenteritis (1.32 mmol/L) an exceptionally low in infants with kwashiorkor (1.10 mmol/L) especially when the latter condition was accompanied by severe diarrhoea (0.66 mmol/L). Hypophosphatemia, as well as hypokalemia, was associated with marked hypotonia. Low levels of serum phosphorus occurred in nine of the 10 malnourished children who died.