This study found that in people with CF: (1) weight gain after six months of ETI treatment depends on pre-treatment BMI values and the presence of CFTR residual function mutations; (2) ETI is highly effective in improving weight gain in subjects with underweight; (3) a significant proportion of the treated population experienced excessive weight gain; (4) weight gain is only moderately correlated with improvement in respiratory function. Here, CFTR is linked to cystic fibrosis.