Background Cystic fibrosis (CF) Is the most common autosomal recessive disease in Caucasian population. More than 900 mutations have been detected in the Cystic Fibrosis Transmembrane Regulator (CFTR) gene. The most common worldwide, is a deletion of phenylalanine 508 (DeltaF508). Aim: To analyze the presence of mutations AT,508, G542X, N1303K, G551D, R553X and S549N in patients from the 5(th) Region of Chile, with a clinical diagnosis of CF. Patients and methods: We studied 17 non-related patients, presenting frequent respiratory tract infections, malabsorption and positive sweat tests, or meconial ileum. Serum immunoglobulins (IgG, IgA, IgM), and total, CD3+ and B-lymphocytes, were determined to discard the presence of an immune deficiency. The molecular study of the gene Was performed by Polymerase Chain Reaction amplification and restriction analysis Results: Immunological parameters were normal In all patients. The DeltaF508 mutation was detected in 11 chromosomes and the mutation G542X In 3 chromosomes. Conclusions: The mutation G542X was the second most frequent mutation found in this sample of Chilean CF patients. Since this mutation has a high frequency In Spanish CF patients, we suggest that this mutation might have bad its origin in Spain.