Cystic fibrosis is an autosomal recessive genetic disorder linked to chromosome 7q in all families studied. Expression of the disease varies, but the genetic basis for clinical heterogeneity is unknown. We describe an extended consanguineous family with pulmonary disease and the sweat gland phenotype of cystic fibrosis. In the members of this family, clinical expression of the disease was mild, as manifested by the absence of severe childhood lung disease and increased longevity with better functional status than that expected for age. The degree of pancreatic exocrine insufficiency varied (4/10), but the older patients had normal pancreatic function. The pedigree suggested the likelihood of common ancestry, and eight of the ten affected persons were clearly related. At least three of the family members with the mildest clinical disease had consanguineous parents and may therefore have been homozygous for a variant cystic fibrosis gene. The mild expression of cystic fibrosis in this family provides evidence for a form of cystic fibrosis that is intrinsically less debilitating than the classic form.