Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) have been shown to cause cystic fibrosis (CF) and male infertility due to congenital bilateral absence of the vas deferens. We report the identification of a 6.8 kb deletion (del14a) and a nonsense mutation (S1455X) in the CFTR genes of a mother and her youngest daughter with isolated elevated sweat chloride concentrations. Detailed clinical evaluation of both individuals found no evidence of pulmonary or pancreatic disease characteristic of CF. A second child in this family with classic CF was homozygous for the del14a mutation, indicating that this mutation caused severe CFTR dysfunction. CFTRmRNA transcripts bearing the S1455X mutation were stable in vivo, implying that this allele encoded a truncated version of CFTR missing the last 26 amino acids. Loss of this region did not affect processing of transiently expressed S1455X-CFTR compared with wild-type CFTR. When expressed in CF airway cells, this mutant generated cAMP-activated whole-cell chloride currents similar to wild-type CFTR. Preservation of chloride channel function of S1455X-CFTR was consistent with normal lung and pancreatic function in the mother and her daughter. These data indicate that mutations in CFTR can be associated with elevated sweat chloride concentrations in the absence of the CF phenotype, and suggest a previously unrecognized functional role in the sweat gland for the C-terminus of CFTR.