In this paper, we describe a novel CF mutation consisting in a C-->T substitution at nucleotide 170 in exon 1 of CFTR gene, converting a serine residue into phenylalanine at position 12 (S13F) of the CFTR protein. This mutation was detected in a single patient of Sardinian descent in compound heterozygosity with T338I, who presented at 9 years of age with hyponatremic dehydration associated with hypochloremia, hypokaliemia and metabolic alkalosis. Sweat chloride values were 120 mmol/l. Now the patient is 11 years old and has not shown so far any pancreatic or lung involvement. The serine to phenylalanine substitution is a non-conservative change, replacing a non polar for a polar amino acid residue and therefore is most likely a disease-causing mutation.