Underweight-for-age (or ‘low weight’) is defined as a weight for age that falls below the 3rd centile. This means that they weigh less than the normal range. Many of these children are ‘failing to thrive’. They appear clinically well and do not look undernourished. They do not have oedema. Unless they are weighed, and their weight is plotted on a centile chart, the diagnosis is frequently missed. Underweight-for-age is the commonest form of malnutrition.