There has been renewed interest in examining the multiple causes of undernutrition and growth failure in cystic fibrosis. It is now recognized that undernutrition is caused by unfavourable energy balance rather than an inherent component of the disease. Furthermore, there appears to be a direct association between the degree of undernutrition and the severity of pulmonary disease, which in turn affects overall prognosis. Energy imbalance may be caused by three main factors: increased energy loss because of nutrient maldigestion; reduced energy intake due to an improper diet and/or anorexia from respiratory disease, abdominal symptoms or clinical depression and increased energy expenditure with advanced lung disease. Most patients are capable of compensating for these factors; provided energy intake is sufficient, normal growth velocity and nutritional status is maintained. However, in a minority of older patients, when advanced lung disease supervenes, energy expenditure rises resulting in an energy deficit. Undernutrition, with loss of energy stores and lean tissue may in turn contribute to progressive deterioration of lung function. When this occurs, long-term invasive methods of nutritional support can restore energy balance.