The total body potassium (TBK) and serum electrolyte concentrations of malnourished children in Cape Town are similar to those in Jamaica. The TBK is almost always low and hyponatraemia is common, but there was no relationship between the TBK and serum Na and K concentrations. These abnormalities, particularly a very low TBK, appear to influence the prognosis. The expression of the observed TBK as a percentage of the expected value for a normal child, of the same weight and height, appears to be a useful index of the severity of the total body potassium deficit.