Cystic fibrosis is inherited in an autosomal recessive pattern, which means two copies of the gene must be altered for a person to be affected by the disorder. Most often, the parents of a child with an autosomal recessive disorder are not affected, but are carriers of one copy of the altered gene.

Mutations in the cystic fibrosis conductance regulator (CFTR) gene cause this disease. The protein made by CFTR controls the movement of salt and water into and out of cells. Mutations in CFTR block this movement, causing cells to produce the abnormally thick mucus characteristic of cystic fibrosis.