Background and aim: despite the knowledge about the importance of adequate nutritional status in the health of patients with cystic fibrosis (CF), achieving protein intake recommendation remains a challenge. The influence of protein on anthropometric parameters in these patients needs to be better studied. The aim of the study was evaluated anthropometric changes after nutritional counselling to increase protein content in CF children and adolescents with low protein intake. Methods: a prospective longitudinal study in 20 children and adolescents with CF. Food record of 3 consecutive days and anthropometric assessment were applied with an interval of 3 months. Patients with a protein intake <20% of the total daily caloric intake received nutritional counselling to increase protein content and patients with protein intake of =20% maintained their routine diet. Data were analyzed using the Generalized Estimating Equations (GEE) model. Results: 15 patients (75%) had a daily protein intake of less than 20% of total daily calories at the first dietary assessment and received nutritional counselling. After 3 months, the protein content of the diet underwent a statistically significant increase, despite the other macronutrients and total daily calories. There was a significant improvement in z scores for weight (p<0.001), height (p<0.001) and body mass index (BMI) (p=0.001) compared to patients with adequate protein intake. Conclusion: most CF patients did not meet the daily recommendation for protein intake. The reach of the proper protein intake allowed an increase in weight, height, and BMI.