Background: Cystic fibrosis (CF) causes malabsorption of nutrients that exacerbate pulmonary problems. Nutritional interventions can improve pulmonary functions. We aimed to evaluate the effects of nutritional intervention in CF patients with malnutrition, and to determine if there is a correlation between nutritional status and pulmonary functions. Methods: The study included 143 CF patients (67 females) with a mean 2 year follow-up time. Patients' sociodemographic data, presenting symptoms and history were recorded. Height-forage , weight-forage , weight-for-length/ height (WFL/H), and body mass index (BMI) were calculated in all patients. Patients were grouped as well nourished, mild malnutrition, moderate malnutrition, and severe malnutrition. These four groups were compared in terms of pulmonary function test results, lung infections, and the hospitalization rate. Results: Among the patients with a WFL/H or BMI z-scores that decreased, the frequency of lung infection was 74.1% and the hospitalization rate was 40.7%, versus 34% and 12.3%, respectively among the patients with a WFL/H or BMI z-scores that increased. The difference was significant (P = 0.02 and P = 0.01, respectively). The difference in bacterial lung infections differed significantly between the four nutritional status groups (P = 0.002). Patients in the well-nourished group had significantly higher pulmonary function test scores than the other groups. The forced expiratory volume in the first second differed significantly between the patients with and without an increase in the WFL/H or BMI z-scores (P = 0.001). Conclusions: The appropriate nutritional intervention to pediatric CF patients with malnutrition, decrease the frequency of lung infections, and improve respiratory function.