There is now sufficient evidence to prove that the maintenance of a high energy normally balanced diet prevents malnutrition/growth retardation and may well enhance prognosis in patients with CF. Most patients are able to tolerate the normal to high fat content without undue problems with steatorrhoea and with the recent advent of more effective enzyme replacement therapy, this should be even less of a problem. Conversely, there is an equally large bulk of evidence to indicate that the maintenance of a low fat diet, while controlling symptoms from steatorrhoea in some patients, is energy depriving and produces growth failure. In children or teenagers who are presently growth retarded, installation of a high energy intake may improve growth in some, but not in others. Further investigation of the latter patients is required to evaluate their persistent anorexia and advantages and disadvantages of nutritional supplementation by invasive techniques. Certainly growth and wellbeing can be vastly improved. However, such studies may well have to be multicentered to obtain sufficient patients to control for the many variables involved, in order to demonstrate the effects on pulmonary function.