Cystic fibrosis (CF) is a multisystem disorder affecting many organs including thelungs, gastrointestinal tract, pancreas and liver. Failure to thrive is a commonpresentation of undiagnosed children and adults diagnosed with CF which mayworsen as the disease progresses.The lung affection is characterized by dehydration of airway surface liquid andimpaired mucociliary clearance. As a result, there is difficulty clearing pathogensfrom the lung, and patients experience chronic pulmonary infections andinflammation. Although CF is a complex disorder affecting many organs, 85% of themortality is a result of lung disease.Nutritional failure remains a common problem for many patients with cystic fibrosis.It is associated with decrease life quality, excess morbidity and poor prognosis.Over the last decade several studies have demonstrated improved clinical outcomeswhen the causes of acute and chronic malnutrition (i.e., inadequate intake, inadequateabsorption and increased requirements) are treated appropriately.Therefore, early detection of malnutrition is critical to allow for timely interventionand rehabilitation.One of the most commonly used parameters to assess nutritional status in adultsincluding CF patients is the body mass index (BMI; weight/height2 kg/m2)). Anadaptation for children has been limited in the past, because BMI is not constantacross the pediatric age range.In recent years, several countries added age- and sex-specific BMI percentiles (BMIP)to their growth charts. BMIP now provide a way of comparing a child’s weight,adjusted for height, with a reference group of the same age, but not necessarily thesame stature. Plotted sequential BMI values can indicate nutritional failure early whenthe pattern changes from a consistent percentile and thus the nutritional status can betracked.BMIP are now widely used to determine the nutritional status of children in healthand disease, but there is still no consensus about its use in children with CF,particularly as the cutpoints for definition of malnutrition remain incompletelydefined.