Malnutrition, including muscle wasting commonly occurs in children with cystic fibrosis (CF), negatively influencing their quality of life and survival. At the time of a diagnosis of CF, severe protein deficits can already be present. It is important to get CF children fed adequately to prevent that their condition becomes worse or that recovery takes longer. Oral supplementation trials showed that gains in lean body mass are difficult to achieve in CF unless specific metabolic abnormalities are targeted. However, the specific needs for certain food components are not clear yet in children that are ill. Therefore, more information is necessary on the need for protein and certain amino acids in children with CF. Previous studies support the concept of essential amino acids (EAA) as an anabolic stimulus in the young and elderly and in insulin resistant states. Until yet no information is present on the anabolic effects of EAA in CF.